main-loop: remove qemu_fd_register(), win32/slirp/socket specific

Open-code the socket registration where it's needed, to avoid
artificially used or unclear generic interface.

Furthermore, the following patches are going to make socket handling use
FD-only inside QEMU, but we need to handle win32 SOCKET from libslirp.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Message-Id: <20230221124802.4103554-12-marcandre.lureau@redhat.com>
diff --git a/net/slirp.c b/net/slirp.c
index 2ee3f1a..0730a93 100644
--- a/net/slirp.c
+++ b/net/slirp.c
@@ -248,7 +248,13 @@
 
 static void net_slirp_register_poll_fd(int fd, void *opaque)
 {
-    qemu_fd_register(fd);
+#ifdef WIN32
+    AioContext *ctxt = qemu_get_aio_context();
+
+    qemu_socket_select(fd, event_notifier_get_handle(&ctxt->notifier),
+                       FD_READ | FD_ACCEPT | FD_CLOSE |
+                       FD_CONNECT | FD_WRITE | FD_OOB, NULL);
+#endif
 }
 
 static void net_slirp_unregister_poll_fd(int fd, void *opaque)