Description of problem: AS of libvirt-0.8.7, libvirt supports IPv6 on its virtual networks (see Bug 514749). virt-manager should also support this. Basically, each virtual network can now have multiple IP addresses, and each of these addresses can be IPv4 or IPv6. DHCP is not supported for IPv6, but autoconf support is automatic (and requires no config).
It is not just that IPv6 is not supported but it cannot handle any network definition that includes IPv6 (with or without DHCPv6).
Gene added this upstream, closing.