title: List all virtualenv
body: <p>In virtualenvwrapper, is there a simple way to list all virtualenv on my machine? </p>
  
  <p>(like what yolk -l does to list all python packages in the current virtual environment?)</p>
  
  <p><strong>CLARIFICATION:</strong> "ls -la" in my env directory does not count. I am looking for a virtualenv or virtualenvwrapper specific command.</p>