vdpa: Mark vdpa_config_ops.get_vq_notification as optional

Since vhost_vdpa_mmap checks for its existence before calling it.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Link: https://lore.kernel.org/r/20211104195248.2088904-1-eperezma@redhat.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>