slirp: replace qemu_set_nonblock()

Replace qemu_set_nonblock() with slirp_set_nonblock()

qemu_set_nonblock() does some event registration with the main
loop. Add a new callback register_poll_fd() for that reason.

Always build the fd-register stub, to avoid #if WIN32.

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