Examples not consistent with 'qemu-vm.nix' conventions

See used labels for automatically identifying filesystems for qemu-vm. [nixpkgs/nixos/modules/virtualisation /qemu-vm.nix](https://github.com/NixOS/nixpkgs/blob/2c3e5ec5df46d3aeee2a1da0bfedd74e21f4bf3a/nixos/modules/virtualisation/qemu-vm.nix#L332C1-L335C41) ``` # Use well-defined and persistent filesystem labels to identify block devices. rootFilesystemLabel = "nixos"; espFilesystemLabel = "ESP"; # Hard-coded by make-disk-image.nix nixStoreFilesystemLabel = "nix-store"; ``` e.g. in [example/simple-efi.nix](https://github.com/nix-community/disko/blob/master/example/simple-efi.nix) the `root` partition should be labeled `nixos` or there should be a general hint to the convention above in the docs.