Comment 5 for bug 862136

From:

https://bbs.archlinux.org/viewtopic.php?id=123855

"In commit 1faa4356a3bd89ea11fb92752d897cff3a20ec0e to the Linux kernel (which was before 2.6.39 got released so this is the first affected version) the behavior of bridges was changed so that they set the carrier state to off if no interfaces are connected."

Libvirt connects a dummy interface to the bridge but doesn't set it to 'up', which means that IPv6 is never activated.