Hello, I hadn't seen the original report on the list, sorry. There is too much traffic on qemu-devel for me to manage to catch these :/
This problem was fixed by e42f869b ("slirp: Make RA build more flexible") and a2f80fdf ("slirp: Send RDNSS in RA only if host has an IPv6 DNS server") which will be included in qemu 2.9. Samuel