MFC r273515, r274055, r274063, r274215, r274065, r274502:

Add VirtIO console driver.

MFC 260847,264055,264867:
- Add a very simple virtio_random(4) driver for FreeBSD guests to harvest
entropy from hypervisors.
- Add support to bhyve for the virtio RNG entropy-source device to provide
entry to bhyve guests.

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

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

Import multiqueue VirtIO net driver from my user/bryanv/vtnetmq branch

This is a significant rewrite of much of the previous driver; lots of
misc. cleanup was also performed, and support for a few other minor
features was also added.

Virtio SCSI driver

Submitted by: Bryan Venteicher bryanv at daemoninthecloset dot org
Reviewed 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