qom: object: move unparenting to the child property's release callback

qom: object: move unparenting to the child property's release callback

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>