slirp: open-code qemu_socket_(un)select()

We are about to make the QEMU socket API use file-descriptor space only,
but libslirp gives us SOCKET as fd, still.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Message-Id: <20230221124802.4103554-14-marcandre.lureau@redhat.com>