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/volume.go

@@ -0,0 +1,78 @@
+package kvm
+
+import (
+	"encoding/xml"
+	"fmt"
+)
+
+const bytesPerGiB = 1 << 30
+
+func overlayVolumeName(name string) string { return name + ".qcow2" }
+
+func seedVolumeName(name string) string { return name + "-seed.iso" }
+
+// volumeSpec describes a storage volume to create. A non-empty BackingPath
+// makes it a copy-on-write overlay of that file.
+type volumeSpec struct {
+	Name          string
+	SizeBytes     int64
+	Format        string
+	BackingPath   string
+	BackingFormat string
+}
+
+func overlayVolume(name string, sizeGB int64, basePath, baseFormat string) volumeSpec {
+	return volumeSpec{
+		Name:          overlayVolumeName(name),
+		SizeBytes:     sizeGB * bytesPerGiB,
+		Format:        formatQcow2,
+		BackingPath:   basePath,
+		BackingFormat: baseFormat,
+	}
+}
+
+func rawVolume(name string, sizeBytes int64) volumeSpec {
+	return volumeSpec{Name: name, SizeBytes: sizeBytes, Format: formatRaw}
+}
+
+// buildVolumeXML renders the volume for StorageVolCreateXML. Capacity is
+// always given in bytes so no libvirt unit-suffix ambiguity (G vs GiB) can
+// creep in.
+func buildVolumeXML(v volumeSpec) (string, error) {
+	x := volumeXML{
+		Type:     "file",
+		Name:     v.Name,
+		Capacity: sizeXML{Unit: "bytes", Value: v.SizeBytes},
+		Target:   volumeTargetXML{Format: formatXML{Type: v.Format}},
+	}
+	if v.BackingPath != "" {
+		x.BackingStore = &backingStoreXML{Path: v.BackingPath, Format: formatXML{Type: v.BackingFormat}}
+	}
+	out, err := xml.Marshal(x)
+	if err != nil {
+		return "", fmt.Errorf("marshalling volume xml: %w", err)
+	}
+	return string(out), nil
+}
+
+type volumeXML struct {
+	XMLName      xml.Name         `xml:"volume"`
+	Type         string           `xml:"type,attr"`
+	Name         string           `xml:"name"`
+	Capacity     sizeXML          `xml:"capacity"`
+	Target       volumeTargetXML  `xml:"target"`
+	BackingStore *backingStoreXML `xml:"backingStore"`
+}
+
+type volumeTargetXML struct {
+	Format formatXML `xml:"format"`
+}
+
+type formatXML struct {
+	Type string `xml:"type,attr"`
+}
+
+type backingStoreXML struct {
+	Path   string    `xml:"path"`
+	Format formatXML `xml:"format"`
+}