slirp: generalize guestfwd with a callback based approach

Instead of calling into QEMU chardev directly, and mixing it with
slirp_add_exec() handling, add a new function slirp_add_guestfwd()
which takes a write callback.

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