qemu_test: Make chown() calls to the store a no-op

The "misc" NixOS test is using Nix to query the store and it tries to
change the ownership of it while doing so.

This fails if Nix is not in a seccomp-sandboxed userid namespace, so
let's make chown() a no-op when applied to store paths.

Fixes the misc test (and possibly future tests) on older Nix versions.

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