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/id.go

@@ -0,0 +1,21 @@
+package kvm
+
+import (
+	"fmt"
+	"strings"
+)
+
+// The providerID carries the host, so Get/Delete stay correct after a
+// placement.zone edit mid-replacement — re-reading the spec then would look
+// at the wrong hypervisor.
+func formatProviderID(host, name string) string {
+	return fmt.Sprintf("hosts/%s/domains/%s", host, name)
+}
+
+func parseProviderID(id string) (host, name string, err error) {
+	parts := strings.Split(id, "/")
+	if len(parts) != 4 || parts[0] != "hosts" || parts[2] != "domains" || parts[1] == "" || parts[3] == "" {
+		return "", "", fmt.Errorf("malformed kvm providerID %q, want hosts/<host>/domains/<name>", id)
+	}
+	return parts[1], parts[3], nil
+}