When creating a virtual network in libvirt, even with ipv6 enabled = true in
virt-manager, or a configuration such as:

<network ipv6='yes'>
  <name>ra2</name>
  <uuid>98e7a9a1-9606-4bf0-b9ed-13bda85e7ee2</uuid>
  <bridge name='virbr2' stp='on' delay='0'/>
  <mac address='52:54:00:72:83:79'/>
  <domain name='ra2'/>
</network>

Ipv6 traffic does not function correctly on the created bridge device. The
cause is that the sysctl for the associated bridge is not correctly configured:

# sysctl -a | grep -i disable_ipv6
...

net.ipv6.conf.virbr2.disable_ipv6 = 1
net.ipv6.conf.virbr2-nic.disable_ipv6 = 0

This sysctl should be set to "0", to allow ipv6 traffic to operate correctly. 

The effect is the out of the box on OpenSUSE leap, ipv6 does not work on virsh
bridge networks. 

Work around is:

sysctl -w net.ipv6.conf.virbr2.disable_ipv6=0