virtio-devices: Consider vhost-user protocol feature as acked

For vhost-user devices, we don't want to loose the vhost-user protocol
feature through the negotiation between guest and device. Since we know
VIRTIO has no knowledge of the vhost-user protocol feature, there is no
way it would ever be acknowledged by the guest. For that reason, we
create each vhost-user device with the set of acked features containing
the vhost-user protocol feature is this one was part of the available
list.

Having the set of acked features containing this bit allows for solving
a bug that was happening through the migration process since the
vhost-user protocol feature wasn't explicitely enabled.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

virtio-devices/src/vhost_user/blk.rs

@@ -167,7 +167,11 @@ impl Blk {
                 device_type: VirtioDeviceType::Block as u32,
                 queue_sizes: vec![vu_cfg.queue_size; num_queues],
                 avail_features: acked_features,
-                acked_features: 0,
+                // If part of the available features that have been acked, the
+                // PROTOCOL_FEATURES bit must be already set through the VIRTIO
+                // acked features as we know the guest would never ack it, thus
+                // the feature would be lost.
+                acked_features: acked_features & VhostUserVirtioFeatures::PROTOCOL_FEATURES.bits(),
                 paused_sync: Some(Arc::new(Barrier::new(2))),
                 min_queues: DEFAULT_QUEUE_NUMBER as u16,
                 ..Default::default()
@@ -288,12 +292,6 @@ impl VirtioDevice for Blk {
 
         let slave_req_handler: Option<MasterReqHandler<SlaveReqHandler>> = None;
 
-        // The backend acknowledged features must contain the protocol feature
-        // bit in case it was initially set but lost through the features
-        // negotiation with the guest.
-        let backend_acked_features = self.common.acked_features
-            | (self.common.avail_features & VhostUserVirtioFeatures::PROTOCOL_FEATURES.bits());
-
         // Run a dedicated thread for handling potential reconnections with
         // the backend.
         let (kill_evt, pause_evt) = self.common.dup_eventfds();
@@ -303,7 +301,7 @@ impl VirtioDevice for Blk {
             queues,
             queue_evts,
             interrupt_cb,
-            backend_acked_features,
+            self.common.acked_features,
             slave_req_handler,
             kill_evt,
             pause_evt,