event_notifier: handle initialization failure better

Add 'initialized' field and use it to avoid touching event notifiers which are
either not initialized or if their initialization failed.

This is somewhat a hack, but it seems the less intrusive way to make
virtio code deal with event notifiers that failed initialization.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20201217150040.906961-4-mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/qemu/event_notifier.h

@@ -24,6 +24,7 @@ struct EventNotifier {
 #else
     int rfd;
     int wfd;
+    bool initialized;
 #endif
 };