slirp: associate slirp_output callback with the Slirp context

Let's make the slirp interface a bit more library-like.
Associate the slirp_output() with a Slirp context.

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