Issue description

I would like to build and run a qemu vm from my current nixos configuration using nixos-rebuild build-vm. The problem is that the generated run-script does not start the vm with a configuration which allows to run hardware demanding applications in it. For this reason I would like to tweak some options which are exposed by the qemu-vm module. But when I do this nixos-rebuild raises errors.

Steps to reproduce

Add:

virtualization.cores = 5;

to /etc/nixos/configuration.nix. And run

$ nixos-rebuild build-vm 
building Nix...
error: The option `virtualisation.cores' defined in `/etc/nixos/configuration.nix' does not exist.
(use '--show-trace' to show detailed location information)
building the system configuration...
these derivations will be built:

Interestingly the run script was built just fine.

Also when I run nixos-rebuild build-vm --fast the error disappears.

Is this a bug or how can I define virtualisation.cores without raising this error on nixos-rebuild ?

Technical details

• system: "x86_64-linux"
• host os: Linux 4.14.110, NixOS, 19.09pre174426.acbdaa569f4 (Loris)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.2
• channels(root): "nixos-19.09pre174426.acbdaa569f4, nixos-hardware, nixpkgs-19.09pre174594.0c0954781e2"
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos