Add KVM provider config: KVMConfig structs and validation

Adds the kvm block to the providers config (SSH access, images map,
machine-type profiles, hosts list) with validation and table tests.
validate() now rejects mismatched type-specific blocks generically via
setBlocks() instead of pairwise checks.

Step 1 of docs/plans/2026-09-04-1404-kvm-provider.md.

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

internal/provider/config.go

@@ -21,6 +21,23 @@ type ProviderConfig struct {
 	Type       string            `json:"type"`
 	Kubernetes *KubernetesConfig `json:"kubernetes,omitempty"`
 	GCP        *GCPConfig        `json:"gcp,omitempty"`
+	KVM        *KVMConfig        `json:"kvm,omitempty"`
+}
+
+// setBlocks names the type-specific blocks that are non-nil, so validate can
+// reject a block that doesn't match Type without a pairwise check per type.
+func (p ProviderConfig) setBlocks() []string {
+	var out []string
+	if p.Kubernetes != nil {
+		out = append(out, "kubernetes")
+	}
+	if p.GCP != nil {
+		out = append(out, "gcp")
+	}
+	if p.KVM != nil {
+		out = append(out, "kvm")
+	}
+	return out
 }
 
 // KubernetesConfig configures the kubernetes-pod provider, which creates
@@ -81,22 +98,28 @@ func (c *Config) validate() error {
 		}
 		seen[p.Name] = true
 
+		prefix := fmt.Sprintf("providers[%d] %q", i, p.Name)
 		switch p.Type {
-		case "kubernetes":
-			if p.GCP != nil {
-				return fmt.Errorf("providers[%d] %q: type is kubernetes but a gcp block is set", i, p.Name)
-			}
-		case "gcp":
-			if p.Kubernetes != nil {
-				return fmt.Errorf("providers[%d] %q: type is gcp but a kubernetes block is set", i, p.Name)
-			}
-			if p.GCP == nil || p.GCP.Project == "" {
-				return fmt.Errorf("providers[%d] %q: gcp.project is required", i, p.Name)
-			}
+		case "kubernetes", "gcp", "kvm":
 		case "":
-			return fmt.Errorf("providers[%d] %q: type is required", i, p.Name)
+			return fmt.Errorf("%s: type is required", prefix)
 		default:
-			return fmt.Errorf("providers[%d] %q: unknown provider type %q", i, p.Name, p.Type)
+			return fmt.Errorf("%s: unknown provider type %q", prefix, p.Type)
+		}
+		for _, block := range p.setBlocks() {
+			if block != p.Type {
+				return fmt.Errorf("%s: type is %s but a %s block is set", prefix, p.Type, block)
+			}
+		}
+		switch p.Type {
+		case "gcp":
+			if p.GCP == nil || p.GCP.Project == "" {
+				return fmt.Errorf("%s: gcp.project is required", prefix)
+			}
+		case "kvm":
+			if err := validateKVM(prefix, p.KVM); err != nil {
+				return err
+			}
 		}
 	}
 	return nil