docs: Add mist.io as libvirt-based application

As reported on the libvirt-users list [1], there's new web
application called mist.io which uses libvirt as one of its
backends. Lets add it into our list of libivrt based
applications.

1: https://www.redhat.com/archives/libvirt-users/2015-February/msg00096.htmlSigned-off-by: NMichal Privoznik <mprivozn@redhat.com>

docs/apps.html.in

@@ -404,6 +404,14 @@
         functions, such as live migration that allows for load
         balancing between cluster nodes, monitoring CPU, memory.
       </dd>
+      <dt><a href="http://mist.io/">mist.io</a></dt>
+      <dd>
+        Mist.io is an open source project and a service that can assist you in
+        managing your virtual machines on a unified way, providing a simple
+        interface for all of your infrastructure (multiple public cloud
+        providers, OpenStack based public/private clouds, Docker servers, bare
+        metal servers and now KVM hypervisors).
+      </dd>
     </dl>
 
     <h2><a name="mobile">Mobile applications</a></h2>