aio: add Win32 implementation

The Win32 implementation will only accept EventNotifiers, thus a few
drivers are disabled under Windows.  EventNotifiers are a good match
for the GSource implementation, too, because the Win32 port of glib
allows to place their HANDLEs in a GPollFD.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/main-loop.c b/main-loop.c
index 1fdc3bd..a86c275 100644
--- a/main-loop.c
+++ b/main-loop.c
@@ -537,6 +537,7 @@
     return aio_poll(qemu_aio_context, true);
 }
 
+#ifdef CONFIG_POSIX
 void qemu_aio_set_fd_handler(int fd,
                              IOHandler *io_read,
                              IOHandler *io_write,
@@ -549,7 +550,6 @@
     qemu_set_fd_handler2(fd, NULL, io_read, io_write, opaque);
 }
 
-#ifdef CONFIG_POSIX
 void qemu_aio_set_event_notifier(EventNotifier *notifier,
                                  EventNotifierHandler *io_read,
                                  AioFlushEventNotifierHandler *io_flush)