nixos/test-instrumentation: Fix eval of boot tests

The boot tests import test-instrumentation.nix directly to create a VM
image that only contains things such as the backdoor and serial console
the same way as used by other NixOS VM tests.

With one difference though: It doesn't need nor want to have 9p
filesystems mounted, because we actually want to test an image rather
than re-using most stuff from the host's store.

Change tested against the boot.uefiUsb and ipv6 tests, just that it
becomes clear we don't break either the tests with 9p nor the boot
tests (which were already broken but now succeed).

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