vmm: Avoid deadlock from waiting on paused device worker threads

A deadlock can happen from the destination VM of live upgrade or
migration due to waiting on paused device worker threads. For example,
when a serialization error happens after the `DeviceManager` struct is
restored (where all virtio device worker threads are spawned but in
paused/parked state), a deadlock will happen from
`DeviceManager::drop()`, as it blocks for waiting worker threads to
join.

This patch ensures that we wake up all device (mostly virtio) worker
threads before we block for them to join.

Signed-off-by: Bo Chen <chen.bo@intel.com>

vmm/src/device_manager.rs

@@ -4977,6 +4977,12 @@ impl BusDevice for DeviceManager {
 
 impl Drop for DeviceManager {
     fn drop(&mut self) {
+        // Wake up the DeviceManager threads (mainly virtio device workers),
+        // to avoid deadlock on waiting for paused/parked worker threads.
+        if let Err(e) = self.resume() {
+            error!("Error resuming DeviceManager: {:?}", e);
+        }
+
         for handle in self.virtio_devices.drain(..) {
             handle.virtio_device.lock().unwrap().shutdown();
         }