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_test.go

@@ -0,0 +1,44 @@
+package kvm
+
+import (
+	"testing"
+
+	"gitea.home.hrajfrisbee.cz/kacerr/egress-proxies-operator/internal/provider"
+)
+
+func TestWithDefaults(t *testing.T) {
+	t.Parallel()
+	cfg := withDefaults(provider.KVMConfig{
+		Images: map[string]provider.KVMImage{
+			"debian": {URL: "https://x/d.qcow2"},
+			"raw":    {URL: "https://x/r.img", Format: "raw"},
+		},
+		Hosts: []provider.KVMHost{
+			{Name: "a", Address: "a:22", User: "virt", Bridge: "br0"},
+			{Name: "b", Address: "b:22", User: "virt", Bridge: "br1", Socket: "/run/s", Pool: "fast"},
+		},
+	})
+	if cfg.DiskSizeGB != defaultDiskSizeGB {
+		t.Errorf("diskSizeGb = %d, want %d", cfg.DiskSizeGB, defaultDiskSizeGB)
+	}
+	if cfg.VendorData != defaultVendorData {
+		t.Errorf("vendorData = %q, want the default", cfg.VendorData)
+	}
+	if cfg.Images["debian"].Format != "qcow2" || cfg.Images["raw"].Format != "raw" {
+		t.Errorf("image formats = %q/%q, want qcow2 default and raw kept", cfg.Images["debian"].Format, cfg.Images["raw"].Format)
+	}
+	if a := cfg.Hosts[0]; a.Socket != defaultSocket || a.Pool != defaultPool {
+		t.Errorf("host a = %+v, want default socket and pool", a)
+	}
+	if b := cfg.Hosts[1]; b.Socket != "/run/s" || b.Pool != "fast" {
+		t.Errorf("host b = %+v, want explicit socket and pool kept", b)
+	}
+}
+
+func TestWithDefaults_keepsExplicitValues(t *testing.T) {
+	t.Parallel()
+	cfg := withDefaults(provider.KVMConfig{DiskSizeGB: 42, VendorData: "#cloud-config\n"})
+	if cfg.DiskSizeGB != 42 || cfg.VendorData != "#cloud-config\n" {
+		t.Errorf("cfg = %+v, want explicit values kept", cfg)
+	}
+}