interrupt: Make notifier function return Option<EventFd>

In anticipation for supporting the notifier function for the legacy
interrupt source group, we need this function to return an EventFd
instead of a reference to this same EventFd.

The reason is we can't return a reference when there's an Arc<Mutex<>>
involved in the call chain.

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

devices/src/legacy/rtc_pl031.rs

@@ -476,8 +476,8 @@ mod tests {
             Ok(())
         }
 
-        fn notifier(&self, _index: InterruptIndex) -> Option<&EventFd> {
-            Some(&self.event_fd)
+        fn notifier(&self, _index: InterruptIndex) -> Option<EventFd> {
+            Some(self.event_fd.try_clone().unwrap())
         }
     }