Introduce a SlirpCb callback to kick the main io-thread.

Add an intermediary sodrop() function that will call SlirpCb.notify
callback when sbdrop() returns true.

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