event_notifier: Set ->initialized earlier in event_notifier_init()

Otherwise the call to event_notifier_set() is a nop, which causes
the SLOF firmware on POWER to hang when booting from a virtio-scsi
device:

virtio_scsi_dataplane_start()
 virtio_scsi_vring_init()
  virtio_bus_set_host_notifier() <- assign == true
   event_notifier_init() <- active == 1
    event_notifier_set() <- fails right away if !e->initialized

Fixes: e34e47eb28 ("event_notifier: handle initialization failure better")
Cc: mlevitsk@redhat.com
Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <20210216120247.1293569-1-groug@kaod.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

util/event_notifier-posix.c

@@ -66,10 +66,10 @@ int event_notifier_init(EventNotifier *e, int active)
         e->rfd = fds[0];
         e->wfd = fds[1];
     }
+    e->initialized = true;
     if (active) {
         event_notifier_set(e);
     }
-    e->initialized = true;
     return 0;
 
 fail: