slirp: Send RDNSS in RA only if host has an IPv6 DNS server

Previously we would always send an RDNSS option in the RA, making the guest
try to resolve DNS through IPv6, even if the host does not actually have
and IPv6 DNS server available.

This makes the RDNSS option enabled only when an IPv6 DNS server is
available.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>