test-driver.py: delete VM state directory after test run

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.

nixos/doc/manual/development/running-nixos-tests-interactively.xml

@@ -38,7 +38,12 @@ starting VDE switch for network 1
  </para>
 
  <para>
-  The machine state is kept across VM restarts in
-  <filename>/tmp/vm-state-</filename><varname>machinename</varname>.
+   You can re-use the VM states coming from a previous run
+   by setting the <command>--keep-vm-state</command> flag.
+<screen>
+<prompt>$ </prompt>./result/bin/nixos-run-vms --keep-vm-state
+</screen>
+  The machine state is stored in the
+  <filename>$TMPDIR/vm-state-</filename><varname>machinename</varname> directory.
  </para>
 </section>