This extracts the testvm configuration into a shared nixosModules.testvm
module that can be reused across different architectures. The module is
now used by both testvm-x86_64 and testvm-aarch64 nixosConfigurations,
eliminating code duplication.
Additionally, nixosConfigurations have been moved to the top-level flake
outputs to follow Nix conventions, rather than being nested inside
eachDefaultSystem. The virtwl proxy reference now uses ${pkgs.system}
instead of hardcoded x86_64-linux to support both architectures.
