nixosTests: fix and re-enable broken aarch64 tests

[Stebalien]

These tests were failing and/or marked broken because:

1. -vga virtio doesn't work on aarch64.
2. Similarly, aarch64 requires EFI.

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

[Stebalien]

This should also fix nixosTests.initrd-secrets-changing and nixosTests.pulseaudio (#495326).

Unfortunately, I have no idea how to run cross-architecture tests locally so I'll leave this as a draft until I can confirm that it actually works.

[Stebalien]

@ofborg test systemd-lock-handler