This question was the same I have found to my issue. It has no answer.
I am running vagrant on my Ubuntu and have ipv6 disabled. When I create an image with vagrant, libvirt returns:
Error while activating network: Call to virNetworkCreate failed: internal error: Failed to apply firewall rules /sbin/ip6tables --table filter --insert FORWARD --in-interface virbr1 --jump REJECT: ip6tables v1.6.1: can't initialize ip6tables table `filter': Address family not supported by protocol Perhaps ip6tables or your kernel needs to be upgraded.
How do I disable ipv6 in libvirt or how do I fix this issue?