AioContext: introduce aio_prepare

This will be used to implement socket polling on Windows.
On Windows, select() and g_poll() are completely different;
sockets are polled with select() before calling g_poll,
and the g_poll must be nonblocking if select() says a
socket is ready.

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

aio-posix.c

@@ -100,6 +100,11 @@ void aio_set_event_notifier(AioContext *ctx,
                        (IOHandler *)io_read, NULL, notifier);
 }
 
+bool aio_prepare(AioContext *ctx)
+{
+    return false;
+}
+
 bool aio_pending(AioContext *ctx)
 {
     AioHandler *node;