tests: use nixpkgs qemu-common for proper machine type configuration

QEMU on aarch64 requires an explicit machine type (e.g., '-machine virt'),
unlike x86_64 which has a default. Previously, the test script hardcoded
'qemu-kvm' without machine type arguments, causing aarch64 tests to fail
with "No machine specified, and there is no default".

By importing and using nixpkgs' qemu-common.nix library, we reuse the
existing platform-specific QEMU configuration logic.

flake.nix

@@ -64,6 +64,7 @@
               inherit pkgs;
               makeTest = import (pkgs.path + "/nixos/tests/make-test-python.nix");
               eval-config = import (pkgs.path + "/nixos/lib/eval-config.nix");
+              qemu-common = import (pkgs.path + "/nixos/lib/qemu-common.nix");
             }
           );