contrib/vhost-user-blk: enable protocol feature for vhost-user-blk

This patch reports the protocol feature that is only advertised by
QEMU if the device implements the config ops.

Signed-off-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>