[libvirt-users] IPV6 configuration

Hi, I am running package libvirt-0.9.11.6-1.fc17.x86_64 on a kernel-3.6.5-1.fc17.x86_64 Fedora 17. I have set up the host with an IPv6 address, and I made some successfull tests transferring in IPv6 mode to/from other hosts. I want to set up libvirt to get an IPv6 address for his local network interface, same for the guests. I want the guests to be able to communicate directly on the internet through this ipv6 address. I have had a <ip family="ipv6" address="2001:xxxxxxx" /> line to the /etc/libvirtd/qemu/network/default.xml file but when I restart the host, this line disappears and no ipv6 address is added to the libvirt interface. I have added an ipv6 address to the guest in the same range of the host address. I can ping6 the address locally on the guest, but can't access the host. Is there somewhere a howto-libvirt-ipv6 to understand how to set up the whole stuff? Thanks for help Patrick