slirp: use polling callbacks, drop glib requirement

It would be legitimate to use libslirp without glib. Let's
add_poll/get_revents pair of callbacks to provide the same
functionality.

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