vhost_user_backend: Don't process disabled queues

Every time an event is triggered, it needs to be read, but only based on
the status of the vring (enabled or not) will decide if the queue needs
to be processed.

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

vhost_user_backend/src/lib.rs

@@ -329,6 +329,12 @@ impl<S: VhostUserBackend> VringEpollHandler<S> {
                         .map_err(VringEpollHandlerError::HandleEventReadKick)?;
                 }
 
+                // If the vring is not enabled, it should not be processed.
+                // The event is only read to be discarded.
+                if !self.vrings[device_event as usize].read().unwrap().enabled {
+                    return Ok(false);
+                }
+
                 self.process_queue(device_event)?;
                 Ok(false)
             }