vhost-user: Fix backends without multiqueue support

dev->max_queues was never initialised for backends that don't support
VHOST_USER_PROTOCOL_F_MQ, so it would use 0 as the maximum number of
queues to check against and consequently fail for any such backend.

Set it to 1 if the backend doesn't have multiqueue support.

Fixes: c90bd50
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20210705171429.29286-1-kwolf@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

hw/virtio/vhost-user.c

@@ -1913,7 +1913,10 @@ static int vhost_user_backend_init(struct vhost_dev *dev, void *opaque,
             if (err < 0) {
                 return -EPROTO;
             }
+        } else {
+            dev->max_queues = 1;
         }
+
         if (dev->num_queues && dev->max_queues < dev->num_queues) {
             error_setg(errp, "The maximum number of queues supported by the "
                        "backend is %" PRIu64, dev->max_queues);