nixos/tests: Use a patched QEMU for testing

The reason to patch QEMU is that with latest Nix, tests like "printing"
or "misc" fail because they expect the store paths to be owned by uid 0
and gid 0.

Starting with NixOS/nix@5e51ffb1c2, Nix
builds inside of a new user namespace. Unfortunately this also means
that bind-mounted store paths that are part of the derivation's inputs
are no longer owned by uid 0 and gid 0 but by uid 65534 and gid 65534.

This in turn causes things like sudo or cups to fail with errors about
insecure file permissions.

So in order to avoid that, let's make sure the VM always gets files
owned by uid 0 and gid 0 and does a no-op when doing a chmod on a store
path.

In addition, this adds a virtualisation.qemu.program option so that we
can make sure that we only use the patched version if we're *really*
running NixOS VM tests (that is, whenever we have imported
test-instrumentation.nix).

Tested against the "misc" and "printing" tests.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

nixos/modules/testing/test-instrumentation.nix

@@ -110,6 +110,9 @@ let kernel = config.boot.kernelPackages.kernel; in
 
     networking.usePredictableInterfaceNames = false;
 
+    # Make sure we use a patched QEMU that ignores file ownership.
+    virtualisation.qemu.program = "${pkgs.qemu_test}/bin/qemu-kvm";
+
     # Make it easy to log in as root when running the test interactively.
     users.extraUsers.root.initialHashedPassword = mkOverride 150 "";