Don't crash when path to executable contains unicode

fixes this error when shutting down the test:

    vnet           : Shutting down nodes...
    Traceback (most recent call last):
      File "./vnet", line 437, in <module>
        main()
      File "./vnet", line 296, in main
        logger.debug("stopping node %s %s" % (name, str(node.vm_instance)))
    UnicodeEncodeError: 'ascii' codec can't encode character u'\xb5' in position 15: ordinal not in range(128)

str() doesn't appear to be needed here