Updating 'Entropy to Instances' section of the Security Guide

Formatted Entropy section and integrated Virtio RNG tool with current
EGD section, and including docbook formatting for files and literals.

Change-Id: I7cdc74a19d6f0ed54763c85936a4a686bb10928a
Closes-Bug: #1348798
Merge "Updating 'Entropy to Instances' section of the Security Guide"