This ensures that the unparent callback is called automatically
when the parent object is finalized.
Note that there's no need to keep a reference neither in
object_unparent nor in object_finalize_child_property. The
reference held by the child property itself will do.
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>