qom: update doc comment for type_register[_static]()

type_register()/type_register_static() functions in current impl.
can't fail returning 0, also none of the users check for error
so update doc comment to reflect current behaviour.

Suggested-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <1507111682-66171-2-git-send-email-imammedo@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
