title: how to detect from a Vagrantfile if a plugin has been installed?
body: <p>I would like to detect if a plugin has been installed, so I can add some useful debugging tips for users.</p>
  
  <p>I have tried the following code:</p>
  
  <pre><code>  if config.vbguest
      config.vbguest.auto_update = true
    else
      puts "installing vagrant-vbguest plugin is recommended"
    end
  </code></pre>
  
  <p>However, Vagrant outputs <code>* Unknown configuration section 'vbguest'.</code></p>
  
  <p>Is there a way I can detect whether a plugin has been installed?</p>