Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC VirtIO r252703,r252707

r252703: Document new cache write mode tunables/sysctl
r252707: Minor cleanup and use my FreeBSD.org email

Also change the release where first appeared closer to reality.

MFC r241470

Add VirtIO SCSI driver man page

Approved by: grehan (mentor)

Virtio SCSI driver

Submitted by: Bryan Venteicher bryanv at daemoninthecloset dot org
Reviewed by: grehan