Simplify the KVM provider after the /simplify review

Collapse duplicated logic the review found in PR #5 without changing
behaviour: one releaseFinalizer helper for the four finalizer exits in
reconcileDelete, classify reusing isClassified and provider.Class, the
image URL check and format/firmware values shared from the provider
package, imageSource and the metadata parser embedding their config
structs instead of copying fields, and the SSH loader as a small struct
that keeps only the SSH fields and is seeded by New instead of loading
the key twice. Tracing builds its V(2)-only lists only at V(2).

Tests: drop a duplicate New test, fold near-identical tests into tables
(connection drops, metadata parsing, cached images), add mustSSHAuth,
and use wantClass where classes were checked by hand.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

internal/provider/kvm/metadata.go

@@ -37,15 +37,13 @@ func newInstanceMetadata(uid string, createdAt time.Time) instanceMetadata {
 // by the lookup, so only the local name is matched here.
 func parseInstanceMetadata(s string) (instanceMetadata, error) {
 	var raw struct {
-		XMLName   xml.Name `xml:"instance"`
-		Managed   string   `xml:"managed"`
-		UID       string   `xml:"uid"`
-		CreatedAt string   `xml:"createdAt"`
+		XMLName xml.Name `xml:"instance"`
+		instanceMetadata
 	}
 	if err := xml.Unmarshal([]byte(s), &raw); err != nil {
 		return instanceMetadata{}, fmt.Errorf("parsing domain metadata: %w", err)
 	}
-	return instanceMetadata{Managed: raw.Managed, UID: raw.UID, CreatedAt: raw.CreatedAt}, nil
+	return raw.instanceMetadata, nil
 }
 
 func (m instanceMetadata) managed() bool {