vm-virtio: queue: add a missing memory barrier in get_used_event

Add a missing memory barrier in get_used_event to make sure we see the
last value written by the guest.

Signed-off-by: Sergio Lopez <slp@redhat.com>

vm-virtio/src/queue.rs

@@ -559,6 +559,8 @@ impl Queue {
                 }
             };
 
+        // This fence ensures we're seeing the latest update from the guest.
+        fence(Ordering::Acquire);
         match mem.read_obj::<u16>(used_event_addr) {
             Ok(ret) => Some(Wrapping(ret)),
             Err(_) => None,