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>

vmm/src/device_manager.rs

@@ -4135,7 +4135,6 @@ impl DeviceManager {
             if let Some(migratable) = &node.migratable {
                 info!("Restoring {} from DeviceManager", node.id);
                 if let Some(snapshot) = snapshot.snapshots.get(&node.id) {
-                    migratable.lock().unwrap().pause()?;
                     migratable.lock().unwrap().restore(*snapshot.clone())?;
                 } else {
                     return Err(MigratableError::Restore(anyhow!(