slirp: use a callback structure to interface with qemu

This will bring slirp a bit forward to the state of an independent
project.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>