test-aio: Fix event notifier cleanup

One test case closed an event notifier (event_notifier_cleanup())
without first disabling it (set_event_notifier(..., NULL)). This
resulted in a leftover handle 0 that was added to each subsequent
WaitForMultipleObjects() call, causing the function to fail (invalid
handle).

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
