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>

qom/object_interfaces.c

@@ -0,0 +1,32 @@
+#include "qom/object_interfaces.h"
+#include "qemu/module.h"
+
+void user_creatable_complete(Object *obj, Error **errp)
+{
+
+    UserCreatableClass *ucc;
+    UserCreatable *uc =
+        (UserCreatable *)object_dynamic_cast(obj, TYPE_USER_CREATABLE);
+
+    if (!uc) {
+        return;
+    }
+
+    ucc = USER_CREATABLE_GET_CLASS(uc);
+    if (ucc->complete) {
+        ucc->complete(uc, errp);
+    }
+}
+
+static void register_types(void)
+{
+    static const TypeInfo uc_interface_info = {
+        .name          = TYPE_USER_CREATABLE,
+        .parent        = TYPE_INTERFACE,
+        .class_size = sizeof(UserCreatableClass),
+    };
+
+    type_register_static(&uc_interface_info);
+}
+
+type_init(register_types)