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>

hw/ppc/spapr_drc.c

@@ -594,6 +594,10 @@ static void spapr_dr_connector_class_init(ObjectClass *k, void *data)
     drck->attach = attach;
     drck->detach = detach;
     drck->release_pending = release_pending;
+    /*
+     * Reason: it crashes FIXME find and document the real reason
+     */
+    dk->cannot_instantiate_with_device_add_yet = true;
 }
 
 static const TypeInfo spapr_dr_connector_info = {