Document required libvirt python bindings

We added a requirement on libvirt python bindings with the script to
retrieve the latest DHCP lease for vagrant developers' environment.

Add it to the vagrant documentation.

Change-Id: I3e7e97922f5b855155fda96b914a4947af007c47

doc/vagrant-dev-env.rst

@@ -38,11 +38,11 @@ choice. Various downloads can be found at the `Vagrant downloads
 
 On Fedora 22 it is as easy as::
 
-    sudo dnf install vagrant ruby-devel libvirt-devel
+    sudo dnf install vagrant ruby-devel libvirt-devel libvirt-python
 
 On Ubuntu 14.04 it is as easy as::
 
-    sudo apt-get install ruby-libvirt libvirt-dev nfs-kernel-server
+    sudo apt-get install ruby-libvirt python-libvirt libvirt-dev nfs-kernel-server
 
 **Note:** Many distros ship outdated versions of Vagrant by default. When in
 doubt, always install the latest from the downloads page above.