vdpa_sim: offer VHOST_BACKEND_F_ENABLE_AFTER_DRIVER_OK

Start offering the feature in the simulator.  Other parent drivers can
follow this code to offer it too.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Acked-by: Shannon Nelson <shannon.nelson@amd.com>
Message-Id: <20230609092127.170673-5-eperezma@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>