virtio-devices: vhost_user: Shutdown communication after migration

During a migration, the vhost-user device talks to the backend to
retrieve the dirty pages. Once done with this, a snapshot will be taken,
meaning there's no need to communicate with the backend anymore. Closing
the communication is needed to let the destination VM being able to
connect to the same backend.

That's why we shutdown the communication with the backend in case a
migration has been started and we're asked for a snapshot.

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

virtio-devices/src/vhost_user/net.rs

@@ -116,6 +116,7 @@ pub struct Net {
     epoll_thread: Option<thread::JoinHandle<()>>,
     seccomp_action: SeccompAction,
     vu_num_queues: usize,
+    migration_started: bool,
 }
 
 impl Net {
@@ -208,6 +209,7 @@ impl Net {
             epoll_thread: None,
             seccomp_action,
             vu_num_queues,
+            migration_started: false,
         })
     }
 
@@ -495,7 +497,13 @@ impl Snapshottable for Net {
     }
 
     fn snapshot(&mut self) -> std::result::Result<Snapshot, MigratableError> {
-        Snapshot::new_from_versioned_state(&self.id(), &self.state())
+        let snapshot = Snapshot::new_from_versioned_state(&self.id(), &self.state())?;
+
+        if self.migration_started {
+            self.shutdown();
+        }
+
+        Ok(snapshot)
     }
 
     fn restore(&mut self, snapshot: Snapshot) -> std::result::Result<(), MigratableError> {
@@ -507,6 +515,7 @@ impl Transportable for Net {}
 
 impl Migratable for Net {
     fn start_dirty_log(&mut self) -> std::result::Result<(), MigratableError> {
+        self.migration_started = true;
         if let Some(vu) = &self.vu {
             if let Some(guest_memory) = &self.guest_memory {
                 let last_ram_addr = guest_memory.memory().last_addr().raw_value();
@@ -530,6 +539,7 @@ impl Migratable for Net {
     }
 
     fn stop_dirty_log(&mut self) -> std::result::Result<(), MigratableError> {
+        self.migration_started = false;
         if let Some(vu) = &self.vu {
             vu.lock().unwrap().stop_dirty_log().map_err(|e| {
                 MigratableError::StopDirtyLog(anyhow!(