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

This reverts commit 704e91bab005eabe968a3b140222fb0cf7afd4db.
Revert "Merge pull request #3182 from wkennington/master.ipv6"

This reverts commit b23fd6585481a42937e105d5fce630a549900e86, reversing
changes made to 43654cba2c280ce17b81db44993d1c1bcae3a9c6.