spapr_drc: fix realize and unrealize

If object_property_add_alias() returns an error in realize(), we should
propagate it to the caller and certainly not unref the DRC.

Same thing goes for unrealize(). Since object_property_del() is the last
call, we can even get rid of the intermediate Error *.

And finally, unrealize() should undo all registrations performed by
realize().

Signed-off-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>