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/balloon.rs

@@ -365,9 +365,14 @@ impl Balloon {
     ) -> io::Result<Self> {
         let mut queue_sizes = vec![QUEUE_SIZE; MIN_NUM_QUEUES];
 
-        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-balloon {}", id);
-            (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;
             if deflate_on_oom {
@@ -382,7 +387,7 @@ impl Balloon {
                 ..Default::default()
             };
 
-            (avail_features, 0, config)
+            (avail_features, 0, config, false)
         };
 
         if free_page_reporting {
@@ -397,6 +402,7 @@ impl Balloon {
                 paused_sync: Some(Arc::new(Barrier::new(2))),
                 queue_sizes,
                 min_queues: MIN_NUM_QUEUES as u16,
+                paused: Arc::new(AtomicBool::new(paused)),
                 ..Default::default()
             },
             id,