Revert "Revert "Merge pull request #3182 from wkennington/master.ipv6""

This reverts commit ea8910652fcecbcd4f21aa1c66b1a0a239408b04.
Revert "Revert "Fix syntax error in nixos/lib/build-vms.nix, introduced by 86c0f8c""

This reverts commit 2f697bf6931b24cdd428e22effbf6427a85afd42.
nixos/network-interfaces: Fix bug in converting old ipAddresses
Merge pull request #3908 from wkennington/master.ip

Reapply the multi-ip code