qa/vstart_runner: minor bug fix and minor adjustments

qa/vstart_runner: minor bug fix and minor adjustments

The message regarding deletion of helper tools is printed for every
command. This message should be printed only when applicable.

Besides -

* Move XXX comments to _do_run() since it increases visibility of
  these messages.
* Move omission of arguments stuff to new method clear up the clutter.
* And remove shell as a parameter from _perform_checks_and_adjustments
  since it's redundant.

Signed-off-by: Rishabh Dave <ridave@redhat.com>