vhost: let SET_VRING_ENABLE message depends on protocol feature

But not depend on PROTOCOL_F_MQ feature bit. So that we could use
SET_VRING_ENABLE to sign the backend on stop, even if MQ is disabled.

That's reasonable, since we will have one queue pair at least.
Signed-off-by: NYuanhan Liu <yuanhan.liu@linux.intel.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/virtio/vhost-user.c

@@ -338,7 +338,7 @@ static int vhost_user_set_vring_enable(struct vhost_dev *dev, int enable)
         .num   = enable,
     };
 
-    if (!(dev->protocol_features & (1ULL << VHOST_USER_PROTOCOL_F_MQ))) {
+    if (!virtio_has_feature(dev->features, VHOST_USER_F_PROTOCOL_FEATURES)) {
         return -1;
     }