Update dev env docs on libvirt-dev(el) requirement

Commit 8f505b85268adc226ec0a83826c2d13edcbe3d7c added
a test requirement on libvirt-python which requires
libvirt-dev(el) being installed, so update the docs
to add that package for the distro.

Change-Id: I7169d3c78a6babe6e20c0dc35874167d0bfee922
Merge "Update dev env docs on libvirt-dev(el) requirement"