Fixes #110: Fix virt view's virt filter

Here's the fix for #110. 

Writing a test for it isn't as trivial. At the moment, the frontend tests use the approach:

````python
self.assertFalse('<img src="/static/img/node_map.svg"' in resp.content)
````

The problem here is that what we need to test here is the difference between:

````
<li>
    <a href="#" data-type="virt" data-name="host" class="sidebar_link">host</a>
</li>
````

and

````
<li class="active">
    <a href="#" data-type="virt" data-name="host" class="sidebar_link">host</a>
</li>
````

The best way to do this would be to properly parse the HTML response, and check the class of the appropriate `<a>` tag's direct parent. I had a go at this with lxml, but it chokes on the response, and I remember you guys not being such fans of BeautifulSoup, so I'll leave this with you ;)