spapr_drc: Make device "spapr-dr-connector" unavailable with -device

It should only be created via spapr_dr_connector_new().  Attempting to
create it with -device crashes.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>