Improve "Hardening the virtualization layer"

Various edits as suggested in bug reports:
* Use variablelist to follow conventions for markup, improve paragraph
  for clarity.
* Clarify a sentence about SELinux.
* Improve CFLAGS recommendation.
* Reword QEMU recommendation.

Change-Id: I28fdb67a9103e3263944a3b22a44cfc16917473e
Closes-Bug: #1342862
Closes-Bug: #1342829
Closes-Bug: #1342826
Closes-Bug: #1342432