xics: Minor fixes for XICSFabric interface

xics: Minor fixes for XICSFabric interface

Interface instances should never be directly dereferenced.  So, the common
practice is to make them incomplete types to make sure no-one does that.
XICSFrabric, however, had a dummy type which is less safe.

We were also using OBJECT_CHECK() where we should have been using
INTERFACE_CHECK().

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