Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

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

Merge virtio changes from projects/virtio

Contains projects/virtio commits:

r245738:
virtio: Minor man page tweaks
r246060:
virtio: Cleanup feature description printing
r246306:
virtio: Remove old debugging flag
r247238:
virtio: Remove PRIx64 macros from format strings
r247239:
virtio: Constify some fields
r247240:
virtio: Minor code simplifications
r249962:
virtio: Update to my freebsd.org email address

MFC after: 1 month

General cleanup.

Prodded by: grehan

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