slirp: Adding IPv6 address for DNS relay

slirp: Adding IPv6 address for DNS relay

This patch adds an IPv6 address to the DNS relay. in6_equal_dns() is
developed using this Slirp attribute.
sotranslate_in/out/accept() are also updated to manage the IPv6 case so the
guest can be able to join the host using one of the Slirp addresses.

For now this only points to localhost. Further development will be needed to
automatically fetch the IPv6 address from resolv.conf, and announce this via
RDNSS.

Signed-off-by: Guillaume Subiron <maethor@subiron.org>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>