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 r252702,r252703,r252704,r252706,r252707,r252708,r252709

This brings in several bug fixes and better conformance with various
aspects of the spec that have changed.

r252702: Convert VirtIO to use ithreads instead of taskqueues
r252703: Block driver bug fixes and spec changes
r252704: Balloon driver improvements
r252706: Minor network driver improvements
r252707: Minor misc VirtIO changes
r252708: PCI driver bug fixes and cleanup
r252709: Fix SCSI driver lock not owned panic

MFC r238072,r238360,r240430,r241469,r241470,r241495,r244136,r244200

Sync VirtIO with HEAD.

- r238072: Do not include <sys/types.h> in the local headers.
- r238360: Various VirtIO improvements
- r240430: No need to leak these into the includer's namespace.
- r241469: virtqueue: Fix non-indirect virtqueues
- r241470: Add Virtio SCSI driver
- r241495: Fix build with PAE enabled
- r244136: Remove duplicated lines
- r244200: Start taskqueues threads after attach cannot fail

Approved by: grehan (mentor)

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