Add KVM provider config: KVMConfig structs and validation

Adds the kvm block to the providers config (SSH access, images map,
machine-type profiles, hosts list) with validation and table tests.
validate() now rejects mismatched type-specific blocks generically via
setBlocks() instead of pairwise checks.

Step 1 of docs/plans/2026-09-04-1404-kvm-provider.md.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

internal/provider/config_kvm.go

@@ -0,0 +1,138 @@
+package provider
+
+import (
+	"fmt"
+	"net/url"
+	"regexp"
+)
+
+// KVMConfig configures a kvm provider instance: one or more libvirt/KVM
+// hypervisors reached over SSH (the libvirt unix socket is forwarded through
+// the SSH connection, so hosts need no libvirtd TCP/TLS listener).
+// placement.zone on a Proxy selects a host by name; with a single host it may
+// be omitted.
+type KVMConfig struct {
+	// SSHKeyFile is the path to the private key used for every host. Required.
+	SSHKeyFile string `json:"sshKeyFile"`
+	// KnownHostsFile is an OpenSSH known_hosts file used to verify host keys.
+	// Required unless InsecureIgnoreHostKey is set.
+	KnownHostsFile string `json:"knownHostsFile,omitempty"`
+	// InsecureIgnoreHostKey skips host-key verification entirely.
+	InsecureIgnoreHostKey bool `json:"insecureIgnoreHostKey,omitempty"`
+	// DiskSizeGB is the capacity of each VM's overlay disk. Default 10. The
+	// guest's cloud-init growpart expands the root filesystem into it.
+	DiskSizeGB int64 `json:"diskSizeGb,omitempty"`
+	// VendorData is cloud-init vendor-data placed on every seed next to the
+	// Proxy's own user-data. Default: installs and starts qemu-guest-agent so
+	// the provider can read the VM's IP even before it shows up in ARP.
+	VendorData string `json:"vendorData,omitempty"`
+	// Images are named base images; placement.image names a key here (or is
+	// an http(s) URL directly). The provider downloads each image into a
+	// host's storage pool on first use and caches it there.
+	Images map[string]KVMImage `json:"images,omitempty"`
+	// MachineTypes are named size profiles; placement.machineType names a
+	// key here. Required, at least one.
+	MachineTypes map[string]KVMMachineType `json:"machineTypes"`
+	// Hosts are the hypervisors. Required, at least one, unique names.
+	Hosts []KVMHost `json:"hosts"`
+}
+
+// KVMImage is one downloadable base image (a distro cloud image with
+// cloud-init preinstalled, e.g. Debian's generic qcow2).
+type KVMImage struct {
+	// URL is where to fetch the image from. Required, http or https.
+	URL string `json:"url"`
+	// SHA256 is the expected hex digest of the file. Optional; when set it is
+	// verified after download and a later change triggers a re-download.
+	SHA256 string `json:"sha256,omitempty"`
+	// Format is the image's disk format, "qcow2" (default) or "raw".
+	Format string `json:"format,omitempty"`
+}
+
+// KVMMachineType is a VM size profile.
+type KVMMachineType struct {
+	VCPUs     int32 `json:"vcpus"`
+	MemoryMiB int64 `json:"memoryMiB"`
+}
+
+// KVMHost is one hypervisor.
+type KVMHost struct {
+	// Name is the placement.zone value that selects this host. Required.
+	Name string `json:"name"`
+	// Address is the SSH endpoint, host:port. Required.
+	Address string `json:"address"`
+	// User is the SSH user; it must be allowed to use the libvirt socket
+	// (typically a member of the libvirt group). Required.
+	User string `json:"user"`
+	// Socket is the libvirt unix socket path on the host.
+	// Default /var/run/libvirt/libvirt-sock.
+	Socket string `json:"socket,omitempty"`
+	// Pool is the libvirt storage pool for base images, overlay disks and
+	// seed ISOs. Default "default".
+	Pool string `json:"pool,omitempty"`
+	// Bridge is the host bridge the VM's NIC attaches to; it must lead to a
+	// network the cluster can reach and that hands out DHCP leases. Required.
+	Bridge string `json:"bridge"`
+}
+
+var sha256Hex = regexp.MustCompile(`^[0-9a-fA-F]{64}$`)
+
+func validateKVM(prefix string, c *KVMConfig) error {
+	if c == nil {
+		return fmt.Errorf("%s: kvm block is required", prefix)
+	}
+	if c.SSHKeyFile == "" {
+		return fmt.Errorf("%s: kvm.sshKeyFile is required", prefix)
+	}
+	if c.KnownHostsFile == "" && !c.InsecureIgnoreHostKey {
+		return fmt.Errorf("%s: kvm.knownHostsFile is required unless kvm.insecureIgnoreHostKey is true", prefix)
+	}
+	if c.DiskSizeGB < 0 {
+		return fmt.Errorf("%s: kvm.diskSizeGb must not be negative", prefix)
+	}
+	for name, img := range c.Images {
+		p := fmt.Sprintf("%s: kvm.images[%q]", prefix, name)
+		u, err := url.Parse(img.URL)
+		if err != nil || (u.Scheme != "http" && u.Scheme != "https") || u.Host == "" {
+			return fmt.Errorf("%s.url must be an http(s) URL (got %q)", p, img.URL)
+		}
+		if img.SHA256 != "" && !sha256Hex.MatchString(img.SHA256) {
+			return fmt.Errorf("%s.sha256 must be 64 hex characters", p)
+		}
+		if img.Format != "" && img.Format != "qcow2" && img.Format != "raw" {
+			return fmt.Errorf("%s.format must be qcow2 or raw (got %q)", p, img.Format)
+		}
+	}
+	if len(c.MachineTypes) == 0 {
+		return fmt.Errorf("%s: kvm.machineTypes must define at least one profile", prefix)
+	}
+	for name, mt := range c.MachineTypes {
+		if mt.VCPUs <= 0 || mt.MemoryMiB <= 0 {
+			return fmt.Errorf("%s: kvm.machineTypes[%q] needs vcpus > 0 and memoryMiB > 0", prefix, name)
+		}
+	}
+	if len(c.Hosts) == 0 {
+		return fmt.Errorf("%s: kvm.hosts must list at least one host", prefix)
+	}
+	seen := make(map[string]bool, len(c.Hosts))
+	for i, h := range c.Hosts {
+		p := fmt.Sprintf("%s: kvm.hosts[%d]", prefix, i)
+		if h.Name == "" {
+			return fmt.Errorf("%s: name is required", p)
+		}
+		if seen[h.Name] {
+			return fmt.Errorf("%s: duplicate host name %q", p, h.Name)
+		}
+		seen[h.Name] = true
+		if h.Address == "" {
+			return fmt.Errorf("%s %q: address is required", p, h.Name)
+		}
+		if h.User == "" {
+			return fmt.Errorf("%s %q: user is required", p, h.Name)
+		}
+		if h.Bridge == "" {
+			return fmt.Errorf("%s %q: bridge is required", p, h.Name)
+		}
+	}
+	return nil
+}