IPv6 not working on KVM instances

On the host:

works.

On a KVM instance:

the IPv6 addresses ipv6.google.com resolves (so the DNS is working) but the ping6 does not go through.

I tried setting the IPv6 gateway of the instance in the Cloudmin Network Interfaces menu for the instance. Interestingly the UI does not save this setting when the instance is shutdown. Is this a bug?

When the instance is running, the UI does save the value, but it disappears when the instance is restarted.

Can you confirm that setting this gateway value should take care of the connectivity?

Note that our Cloudmin master is running on the same host that is hosting the guests.

IPv6 is very useful for us for testing - it is becoming difficult to get IPv4 addresses and we use the few we have for production servers and like to use the IPv6 addresses for test servers without IPv4.