For leaf classes, in many cases the callbacks will simply downcast
the object back to the original class.  Add this fast path to
object_class_dynamic_cast, object_dynamic_cast will inherit it.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Message-id: 1368188203-3407-4-git-send-email-pbonzini@redhat.com
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>