Add KVM provider pure builders: domain XML, volumes, seed ISO, metadata

Introduces internal/provider/kvm with the side-effect-free pieces of the
provider: providerID format/parse, the namespaced <metadata> element that
carries the GC contract, q35 domain XML over pool volumes, volume XML for
overlays and raw seeds, the NoCloud seed ISO built with kdomanski/iso9660,
and config defaults. All unit-tested.

Step 2 of docs/plans/2026-09-04-1404-kvm-provider.md.

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

internal/provider/kvm/kvm.go

@@ -0,0 +1,47 @@
+// Package kvm implements the provider contract on libvirt/KVM hypervisors.
+// Each configured host is reached over SSH with the libvirt unix socket
+// forwarded through the connection, so the hypervisors need no libvirtd TCP
+// or TLS listener and the operator image needs no libvirt libraries. VMs
+// boot a stock cloud image (fetched into the host's storage pool on first
+// use) through a qcow2 overlay, with the Proxy's cloud-init delivered on a
+// NoCloud seed ISO built in-process.
+package kvm
+
+import "gitea.home.hrajfrisbee.cz/kacerr/egress-proxies-operator/internal/provider"
+
+const (
+	defaultDiskSizeGB = 10
+	defaultSocket     = "/var/run/libvirt/libvirt-sock"
+	defaultPool       = "default"
+	formatQcow2       = "qcow2"
+	formatRaw         = "raw"
+)
+
+func withDefaults(cfg provider.KVMConfig) provider.KVMConfig {
+	if cfg.DiskSizeGB == 0 {
+		cfg.DiskSizeGB = defaultDiskSizeGB
+	}
+	if cfg.VendorData == "" {
+		cfg.VendorData = defaultVendorData
+	}
+	images := make(map[string]provider.KVMImage, len(cfg.Images))
+	for k, img := range cfg.Images {
+		if img.Format == "" {
+			img.Format = formatQcow2
+		}
+		images[k] = img
+	}
+	cfg.Images = images
+	hosts := make([]provider.KVMHost, len(cfg.Hosts))
+	for i, h := range cfg.Hosts {
+		if h.Socket == "" {
+			h.Socket = defaultSocket
+		}
+		if h.Pool == "" {
+			h.Pool = defaultPool
+		}
+		hosts[i] = h
+	}
+	cfg.Hosts = hosts
+	return cfg
+}