virtio-devices: Enforce a minimum number of queues

Even though the driver can provide fewer queues than those advertised
for some device types their is a minimum number that is required for
operation.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

virtio-devices/src/vhost_user/blk.rs

@@ -135,6 +135,7 @@ impl Blk {
                 avail_features,
                 acked_features,
                 paused_sync: Some(Arc::new(Barrier::new(vu_cfg.num_queues + 1))),
+                min_queues: 1,
                 ..Default::default()
             },
             id,

virtio-devices/src/vhost_user/fs.rs

@@ -352,6 +352,7 @@ impl Fs {
                 acked_features,
                 queue_sizes: vec![queue_size; num_queues],
                 paused_sync: Some(Arc::new(Barrier::new(2))),
+                min_queues: NUM_QUEUE_OFFSET as u16,
                 ..Default::default()
             },
             id,

virtio-devices/src/vhost_user/net.rs

@@ -144,6 +144,7 @@ impl Net {
                 avail_features,
                 acked_features,
                 paused_sync: Some(Arc::new(Barrier::new((vu_cfg.num_queues / 2) + 1))),
+                min_queues: 2,
                 ..Default::default()
             },
             vhost_user_net,