Update devref setup docs for latest libvirt on ubuntu

Adds instructions on how to enable the ubuntu cloud-archive
and update to the latest libvirt-dev from the icehouse repo.

This is needed to run unit tests in Juno if you're still on
ubuntu precise.

Change-Id: I8c93afdd72256b2bdc487e1413bdd570a6ea59e8
Merge "Update devref setup docs for latest libvirt on ubuntu"