For a lot of the work the non-interactive drivers are enough and it is
probably a good idea to keep it accessible for debugging without
touching the Nix expression.
Keeping the VM state test across several run sometimes lead to subtle
and hard to spot errors in practice. We delete the VM state which
contains (among other things) the qcow volume.
We also introduce a -K (--keep-vm-state) flag making VM state to
persist after the test run. This flag makes test-driver.py to match
its previous behaviour.
This fixes some quirks I introduced in previous commits.
1. No need for an extra newline when printing the output of shell commands.
2. 'or die' is what's already used in the NixOS test sources, while
'die unless' has no occurrences.