add optional 2nd stage initialization to -object/object-add commands

Introduces USER_CREATABLE interface that must be implemented by
objects which are designed to created with -object CLI option or
object-add QMP command.

Interface provides an ability to do an optional second stage
initialization of the object created with -object/object-add
commands. By providing complete() callback, which is called
after the object properties were set.

It allows to:
 * prevents misusing of -object/object-add by filtering out
   objects that are not designed for it.
 * generalize second stage backend initialization instead of
   adding custom APIs to perform it
 * early error detection of backend initialization at -object/
   object-add time rather than through a proxy DEVICE object
   that tries to use backend.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

include/qom/object_interfaces.h

@@ -0,0 +1,62 @@
+#ifndef OBJECT_INTERFACES_H
+#define OBJECT_INTERFACES_H
+
+#include "qom/object.h"
+
+#define TYPE_USER_CREATABLE "user-creatable"
+
+#define USER_CREATABLE_CLASS(klass) \
+     OBJECT_CLASS_CHECK(UserCreatableClass, (klass), \
+                        TYPE_USER_CREATABLE)
+#define USER_CREATABLE_GET_CLASS(obj) \
+     OBJECT_GET_CLASS(UserCreatableClass, (obj), \
+                      TYPE_USER_CREATABLE)
+#define USER_CREATABLE(obj) \
+     INTERFACE_CHECK(UserCreatable, (obj), \
+                     TYPE_USER_CREATABLE)
+
+
+typedef struct UserCreatable {
+    /* <private> */
+    Object Parent;
+} UserCreatable;
+
+/**
+ * UserCreatableClass:
+ * @parent_class: the base class
+ * @complete: callback to be called after @obj's properties are set.
+ *
+ * Interface is designed to work with -object/object-add/object_add
+ * commands.
+ * Interface is mandatory for objects that are designed to be user
+ * creatable (i.e. -object/object-add/object_add, will accept only
+ * objects that inherit this interface).
+ *
+ * Interface also provides an optional ability to do the second
+ * stage * initialization of the object after its properties were
+ * set.
+ *
+ * For objects created without using -object/object-add/object_add,
+ * @user_creatable_complete() wrapper should be called manually if
+ * object's type implements USER_CREATABLE interface and needs
+ * complete() callback to be called.
+ */
+typedef struct UserCreatableClass {
+    /* <private> */
+    InterfaceClass parent_class;
+
+    /* <public> */
+    void (*complete)(UserCreatable *uc, Error **errp);
+} UserCreatableClass;
+
+/**
+ * user_creatable_complete:
+ * @obj: the object whose complete() method is called if defined
+ * @errp: if an error occurs, a pointer to an area to store the error
+ *
+ * Wrapper to call complete() method if one of types it's inherited
+ * from implements USER_CREATABLE interface, otherwise the call does
+ * nothing.
+ */
+void user_creatable_complete(Object *obj, Error **errp);
+#endif