util/aio-win32: Correct the event array size in aio_poll()

util/aio-win32: Correct the event array size in aio_poll()

WaitForMultipleObjects() can only wait for MAXIMUM_WAIT_OBJECTS
object handles. Correct the event array size in aio_poll() and
add a assert() to ensure it does not cause out of bound access.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20221019102015.2441622-3-bmeng.cn@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>