qemu: add a cleanup callback function to EventNotifier

Adding a cleanup callback function to the EventNotifier struct
which allows users to execute event_notifier_cleanup in a
different context.

Signed-off-by: Gal Hammer <ghammer@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>