docs: mention vagrant-libvirt in apps.html

Doc patch for apps.html as per: http://libvirt.org/apps.html#add

Disclaimer: I've contributed patches to the project that this commit
adds.

Vagrant-Libvirt is an excellent way to use vagrant with libvirt. This
way you can benefit from the vagrant features, while not loosing access
to the familiar (and useful) tools such as virsh and virt-manager.

Development currently at:

https://github.com/pradels/vagrant-libvirt/

although recent contributors include:

https://github.com/sciurus/vagrant-libvirt/

and:

https://github.com/purpleidea/vagrant-libvirt/

see git log for more details.
Signed-off-by: NEric Blake <eblake@redhat.com>

docs/apps.html.in

@@ -110,6 +110,12 @@
         application stap can use libvirt to gather data within virtual
         machines.
       </dd>
+      <dt><a href="https://github.com/pradels/vagrant-libvirt/">vagrant-libvirt</a></dt>
+      <dd>
+        Vagrant-Libvirt is a Vagrant plugin that uses libvirt to manage virtual
+        machines. It is a command line tool for developers that makes it very
+        fast and easy to deploy and re-deploy an environment of vm's.
+      </dd>
     </dl>
 
     <h2><a name="configmgmt">Configuration Management</a></h2>