nixosTests.qemu-vm-external-disk-image: fix test (#420333)

Changed files
+2
nixos
+2
nixos/tests/qemu-vm-external-disk-image.nix
···
"/".device = rootFsDevice;
};
system.build.diskImage = import ../lib/make-disk-image.nix {
inherit config lib pkgs;
label = rootFslabel;
···
"/".device = rootFsDevice;
};
+
system.switch.enable = true;
+
system.build.diskImage = import ../lib/make-disk-image.nix {
inherit config lib pkgs;
label = rootFslabel;