Commit '04cb819' incorrectly removed all documentation related to the
sriov-agent. The only thing that has changed with this agent in recent
releases is that it is now required, rather than optional. Some of this
change has already been reverted in '04cb819'. Revert the remainder.
Change-Id: I6bdd5b8d7e8850e23c3dd47161b2baee110b045b
Related-bug: #1508557