vdpa: add get_backend_features vdpa operation

This operation allow vdpa parent to expose its own backend feature bits.

Next patches introduce a feature not compatible with all parent drivers:
the ability to enable vq after driver_ok.  Each parent must declare if
it allows it or not.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Acked-by: Shannon Nelson <shannon.nelson@amd.com>
Message-Id: <20230609092127.170673-4-eperezma@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>