kvm2: config block, constructor registration, IDs, state mapping and classify

The kvm2 block reuses KVMConfig; validateKVM gains a block-name variant so
messages say kvm2. The package skeleton has a non-dialing New, strict
providerID parsing, the state table and a 34-row classify test.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

internal/provider/kvm2/id.go

@@ -0,0 +1,28 @@
+package kvm2
+
+import (
+	"strings"
+)
+
+// The providerID is hosts/<host>/domains/<name>. Host names are validated at
+// config load to contain no slash, and domain names come from NameFromUID,
+// so a strict four-segment split is enough to parse it.
+const (
+	idHostsSegment   = "hosts"
+	idDomainsSegment = "domains"
+)
+
+func formatProviderID(host, name string) string {
+	return idHostsSegment + "/" + host + "/" + idDomainsSegment + "/" + name
+}
+
+// parseProviderID splits an ID back into host and domain name. Anything
+// that is not exactly hosts/<host>/domains/<name> with both non-empty is
+// ErrPermanent: no retry will make it resolve.
+func parseProviderID(id string) (host, name string, err error) {
+	parts := strings.Split(id, "/")
+	if len(parts) != 4 || parts[0] != idHostsSegment || parts[2] != idDomainsSegment || parts[1] == "" || parts[3] == "" {
+		return "", "", permanentf("malformed providerID %q: want hosts/<host>/domains/<name>", id)
+	}
+	return parts[1], parts[3], nil
+}