anchor.diff:  
       virtualisation.qemu.drives = mkMerge [
         [{
  +        name = "root";
           file = "$NIX_DISK_IMAGE";
           driveExtraOpts.cache = "writeback";
           driveExtraOpts.werror = "report";
         }]
         (mkIf cfg.useBootLoader [
           {
  +          name = "boot";
             file = "$TMPDIR/disk.img";
             driveExtraOpts.media = "disk";
             deviceExtraOpts.bootindex = "1";
  
positive.code:         mkOption {
            type = types.listOf (types.submodule driveOpts);
            description = "Drives passed to qemu.";
          };
  
        diskInterface =
  
negative.code:       "/dev/sd${letter}"
      else
        "/dev/vd${letter}";
  
    lookupDriveDeviceName = driveName: driveList:
      (findSingle (drive: drive.name == driveName)
        (throw "Drive ${driveName} not found")
        (throw "Multiple drives named ${driveName}") driveList).device;
  
    addDeviceNames =
      imap1 (idx: drive: drive // { device = driveDeviceName idx; });
  
    # Shell script to start the VM.
    startVM =
  