Add an ObjectClass method that is done at object_unparent time.  It
should remove any backlinks to the object in the composition tree,
so that object_delete will be able to drop the last reference and
free the object.

Use it for qdev buses.
Reviewed-by: NAndreas Färber <afaerber@suse.de>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>