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 part of r241470 missed in r246582

Add the VirtIO SCSI Makefile. I was in one directory too
deep when I did the previous commit.

Approved by: grehan

MFC r227652, r228301, r234270, r234349

Sync Bryan Venteicher's virtio base, PCI front-end, and net/block/balloon
drivers from HEAD.

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