virtio-devices, vmm: Always restore virtio devices in paused state

Following the new restore design, it is not appropriate to set every
virtio device threads into a paused state after they've been started.

This is why we remove the line of code pausing the devices only after
they've been restored, and replace it with a small patch in every virtio
device implementation. When a virtio device is created as part of a
restored VM, the associated "paused" boolean is set to true. This
ensures the corresponding thread will be directly parked when being
started, avoiding the thread to be in a different state than the one it
was on the source VM during the snapshot.

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

virtio-devices/src/mem.rs

@@ -736,10 +736,15 @@ impl Mem {
             ));
         }
 
-        let (avail_features, acked_features, config) = if let Some(state) = state {
+        let (avail_features, acked_features, config, paused) = if let Some(state) = state {
             info!("Restoring virtio-mem {}", id);
             *(blocks_state.lock().unwrap()) = state.blocks_state.clone();
-            (state.avail_features, state.acked_features, state.config)
+            (
+                state.avail_features,
+                state.acked_features,
+                state.config,
+                true,
+            )
         } else {
             let mut avail_features = 1u64 << VIRTIO_F_VERSION_1;
 
@@ -779,7 +784,7 @@ impl Mem {
                 )
             })?;
 
-            (avail_features, 0, config)
+            (avail_features, 0, config, false)
         };
 
         let host_fd = region
@@ -794,6 +799,7 @@ impl Mem {
                 paused_sync: Some(Arc::new(Barrier::new(2))),
                 queue_sizes: QUEUE_SIZES.to_vec(),
                 min_queues: 1,
+                paused: Arc::new(AtomicBool::new(paused)),
                 ..Default::default()
             },
             id,