vmm, vm-virtio: Restore DeviceManager's devices in a paused state

The same way the VM and the vCPUs are restored in a paused state, all
devices associated with the device manager must be restored in the same
paused state.

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

vm-virtio/src/vhost_user/handler.rs

@@ -117,6 +117,14 @@ impl<S: VhostUserMasterReqHandler> VhostUserEpollHandler<S> {
 
         let mut events = vec![epoll::Event::new(epoll::Events::empty(), 0); index + 1];
 
+        // Before jumping into the epoll loop, check if the device is expected
+        // to be in a paused state. This is helpful for the restore code path
+        // as the device thread should not start processing anything before the
+        // device has been resumed.
+        while paused.load(Ordering::SeqCst) {
+            thread::park();
+        }
+
         'poll: loop {
             let num_events = match epoll::wait(epoll_file.as_raw_fd(), -1, &mut events[..]) {
                 Ok(res) => res,