fix(vm): explicitly allow "" as a value for CloudInit interfaces (#546)

The CloudInit interface can be left empty in order to allow
autodetection of the drive being used. However, it would seem that this
value was causing problems (see #539).

This commit adds an additional validator for CloudInit interfaces which
allows the `""` value.

proxmoxtf/resource/validator/vm.go

@@ -253,6 +253,18 @@ func IDEInterface() schema.SchemaValidateDiagFunc {
 	}, false))
 }
 
+// CloudInitInterface is a schema validation function that accepts either an IDE interface identifier or an
+// empty string, which is used as the default and means "detect which interface should be used automatically".
+func CloudInitInterface() schema.SchemaValidateDiagFunc {
+	return validation.ToDiagFunc(validation.StringInSlice([]string{
+		"",
+		"ide0",
+		"ide1",
+		"ide2",
+		"ide3",
+	}, false))
+}
+
 // CloudInitType is a schema validation function for cloud-init types.
 func CloudInitType() schema.SchemaValidateDiagFunc {
 	return validation.ToDiagFunc(validation.StringInSlice([]string{

proxmoxtf/resource/vm.go

@@ -795,7 +795,7 @@ func VM() *schema.Resource {
 							Description:      "The IDE interface on which the CloudInit drive will be added",
 							Optional:         true,
 							Default:          dvResourceVirtualEnvironmentVMInitializationInterface,
-							ValidateDiagFunc: validator.IDEInterface(),
+							ValidateDiagFunc: validator.CloudInitInterface(),
 							DiffSuppressFunc: func(k, oldValue, newValue string, d *schema.ResourceData) bool {
 								return newValue == ""
 							},