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.

man4 MFC r227652 and r230450

man pages were missed in the MFC of virtio(4) to stable/9

r233648 needs to be applied as well as r241470

Import virtio base, PCI front-end, and net/block/balloon drivers.
Tested on Qemu/KVM, VirtualBox, and BHyVe.

Currently built as modules-only on i386/amd64. Man pages not yet hooked
up, pending review.

Submitted by: Bryan Venteicher bryanv at daemoninthecloset dot org
Reviewed by: bz
MFC after: 4 weeks or so