vmstate_register_with_alias_id: Take an Error **

I'll be adding an error to it in a subsequent patch.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Message-Id: <20170202125956.21942-2-dgilbert@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>