virtio-devices: vhost_user: Support inflight I/O tracking

Vhost user INFLIGHT_SHMFD protocol feature supports inflight I/O
tracking, this commit implement the vhost-user device (master) support
of the feature. Till this commit, specific vhost-user devices (blk, fs,
or net) have not enable this feature.

Signed-off-by: Jiachen Zhang <zhangjiachen.jaycee@bytedance.com>

virtio-devices/src/vhost_user/blk.rs

@@ -231,6 +231,7 @@ impl VirtioDevice for Blk {
             &interrupt_cb,
             backend_acked_features,
             &slave_req_handler,
+            None,
         )
         .map_err(ActivateError::VhostUserBlkSetup)?;
 
@@ -251,6 +252,7 @@ impl VirtioDevice for Blk {
             socket_path: self.socket_path.clone(),
             server: false,
             slave_req_handler: None,
+            inflight: None,
         };
 
         let paused = self.common.paused.clone();