slirp: unregister the win32 SOCKET

Presumably, this is what should happen when the SOCKET is to be removed.
(it probably worked until now because closesocket() does it implicitly,
but we never now how the slirp library could use the SOCKET later)

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Message-Id: <20230221124802.4103554-13-marcandre.lureau@redhat.com>

net/slirp.c

@@ -259,7 +259,9 @@ static void net_slirp_register_poll_fd(int fd, void *opaque)
 
 static void net_slirp_unregister_poll_fd(int fd, void *opaque)
 {
-    /* no qemu_fd_unregister */
+#ifdef WIN32
+    qemu_socket_unselect(fd, NULL);
+#endif
 }
 
 static void net_slirp_notify(void *opaque)