aio/win32: aio_set_fd_handler() only supports SOCKET

Let's check if the argument is actually a SOCKET, else report an error
and return.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Message-Id: <20230221124802.4103554-10-marcandre.lureau@redhat.com>