server-config/hardware-configuration.nix

{ modulesPath, ... }:
{
  imports = [ "${modulesPath}/profiles/qemu-guest.nix" ];
  boot.loader.grub.device = "/dev/vda";
  fileSystems."/" = { device = "/dev/vda1"; fsType = "ext4"; };
  swapDevices = [
    {
      device = "/var/lib/swapfile";
      size = 2*1024;
    }
  ];
}