slirp: replace global polling with per-instance & notifier

Remove hard-coded dependency on slirp in main-loop, and use a "poll"
notifier instead. The notifier is registered per slirp instance.

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