nixos/tests: make sure we use the qemu_test package to provide the Guest Agent
nixos/tests: make sure we use the qemu_test package to provide the Guest Agent

This reduces the closure size for the minimal test by a lot since we no
longer have to build the regular QEMU for even the simplest test.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs