win32: work around main-loop busy loop on socket/fd event

Commit 05e514b1d4 introduced an AIO
context optimization to avoid calling event_notifier_test_and_clear() on
ctx->notifier. On Windows, the same notifier is being used to wakeup the
wait on socket events (see commit
d3385eb448).

The ctx->notifier event is added to the gpoll sources in
aio_set_event_notifier(), aio_ctx_check() should clear the event
regardless of ctx->notified, since Windows sets the event by itself,
bypassing the aio->notified. This fixes qemu not clearing the event
resulting in a busy loop.

Paolo suggested to me on irc to call event_notifier_test_and_clear()
after select() >0 from aio-win32.c's aio_prepare. Unfortunately, not all
fds associated with ctx->notifiers are in AIO fd handlers set.
(qemu_set_nonblock() in util/oslib-win32.c calls qemu_fd_register()).

This is essentially a v2 of a patch that was sent earlier:
https://lists.gnu.org/archive/html/qemu-devel/2017-01/msg00420.html

that resurfaced when James investigated Spice performance issues on Windows:
https://gitlab.freedesktop.org/spice/spice/issues/36

In order to test that patch, I simply tried running test-char on
win32, and it hangs. Applying that patch solves it. QIO idle sources
are not dispatched. I haven't investigated much further, I suspect
source priorities and busy looping still come into play.

This version keeps the "notified" field, so event_notifier_poll()
should still work as expected.

Cc: James Le Cuirot <chewi@gentoo.org>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

util/async.c

@@ -354,7 +354,11 @@ void aio_notify(AioContext *ctx)
 
 void aio_notify_accept(AioContext *ctx)
 {
-    if (atomic_xchg(&ctx->notified, false)) {
+    if (atomic_xchg(&ctx->notified, false)
+#ifdef WIN32
+        || true
+#endif
+    ) {
         event_notifier_test_and_clear(&ctx->notifier);
     }
 }