The event notifier state is only reset by test_and_clear. But we created
the windows event object with auto-reset, which subtly swallowed events.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>