nixos/qemu-vm: add option to use a non-standard BIOS

I'd like to change the default on x86 platforms to qboot at some point, since it
saves a fair bit of startup time.

pkgs/applications/virtualization/qboot/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, meson, ninja, fetchFromGitHub }:
+{ stdenv, meson, ninja, fetchFromGitHub, nixosTests }:
 
 stdenv.mkDerivation {
   name = "qboot-20200423";
@@ -19,6 +19,8 @@ stdenv.mkDerivation {
 
   hardeningDisable = [ "stackprotector" "pic" ];
 
+  passthru.tests = { qboot = nixosTests.qboot; };
+
   meta = {
     description = "A simple x86 firmware for booting Linux";
     homepage = "https://github.com/bonzini/qboot";