nixos/tests/vbox: Make shutdown less noisy

Using waitUntilSucceeds for testing whether the shutdown signalling
files have vanished is quite noisy because it prints two lines for every
try. This is now fixed with a while loop on the guest VM which does the
same check but with only one output for the command that's executed and
another one when the conditions are met.

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