`nix run github:nix-community/nixos-generators -- -c default.nix -f vm -o result/vm`
This will build the VM, and register the result in the `result/vm` symlink.
To run the VM, use `sudo result/vm/bin/run-qemu-vm`. (sudo can probably be avoided if you are in the KVM group... I think. TODO Lambda look this up)
You can combine the two steps with the following: `sudo $(nix run github:nix-community/nixos-generators -- -c default.nix -f vm -o result/vm)`
Or to build and run without registering a link (Good if you want a throw away VM which can be garbage collected): `sudo $(nix run github:nix-community/nixos-generators -- -c default.nix -f vm)`