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>

vm-device/src/interrupt/mod.rs

@@ -169,7 +169,7 @@ pub trait InterruptSourceGroup: Send + Sync {
     /// to inject interrupts into a guest, by writing to the file returned
     /// by this method.
     #[allow(unused_variables)]
-    fn notifier(&self, index: InterruptIndex) -> Option<&EventFd> {
+    fn notifier(&self, index: InterruptIndex) -> Option<EventFd> {
         // One use case of the notifier is to implement vhost user backends.
         // For all other implementations we can just return None here.
         None