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_test.go

@@ -0,0 +1,62 @@
+package kvm
+
+import (
+	"encoding/xml"
+	"strings"
+	"testing"
+)
+
+func TestVolumeNames(t *testing.T) {
+	t.Parallel()
+	if got := overlayVolumeName("proxy-x"); got != "proxy-x.qcow2" {
+		t.Errorf("overlayVolumeName = %q", got)
+	}
+	if got := seedVolumeName("proxy-x"); got != "proxy-x-seed.iso" {
+		t.Errorf("seedVolumeName = %q", got)
+	}
+}
+
+func TestBuildVolumeXML_overlay(t *testing.T) {
+	t.Parallel()
+	raw, err := buildVolumeXML(overlayVolume("proxy-x", 10, "/var/lib/libvirt/images/base-debian-13.qcow2", "qcow2"))
+	if err != nil {
+		t.Fatalf("buildVolumeXML() error = %v", err)
+	}
+	var v volumeXML
+	if err := xml.Unmarshal([]byte(raw), &v); err != nil {
+		t.Fatalf("output is not well-formed XML: %v", err)
+	}
+	if v.Type != "file" || v.Name != "proxy-x.qcow2" {
+		t.Errorf("type/name = %s/%s", v.Type, v.Name)
+	}
+	if v.Capacity.Unit != "bytes" || v.Capacity.Value != 10*bytesPerGiB {
+		t.Errorf("capacity = %d %s, want %d bytes", v.Capacity.Value, v.Capacity.Unit, 10*bytesPerGiB)
+	}
+	if v.Target.Format.Type != "qcow2" {
+		t.Errorf("format = %q", v.Target.Format.Type)
+	}
+	if v.BackingStore == nil {
+		t.Fatalf("backingStore missing:\n%s", raw)
+	}
+	if v.BackingStore.Path != "/var/lib/libvirt/images/base-debian-13.qcow2" || v.BackingStore.Format.Type != "qcow2" {
+		t.Errorf("backingStore = %+v", v.BackingStore)
+	}
+}
+
+func TestBuildVolumeXML_rawHasNoBackingStore(t *testing.T) {
+	t.Parallel()
+	raw, err := buildVolumeXML(rawVolume("proxy-x-seed.iso", 372736))
+	if err != nil {
+		t.Fatalf("buildVolumeXML() error = %v", err)
+	}
+	if strings.Contains(raw, "backingStore") {
+		t.Errorf("raw volume must not declare a backing store:\n%s", raw)
+	}
+	var v volumeXML
+	if err := xml.Unmarshal([]byte(raw), &v); err != nil {
+		t.Fatalf("output is not well-formed XML: %v", err)
+	}
+	if v.Name != "proxy-x-seed.iso" || v.Capacity.Value != 372736 || v.Target.Format.Type != "raw" {
+		t.Errorf("volume = %+v", v)
+	}
+}