MFC 308664,308742,308743

308664
hyperv/vss: Add driver and tools for VSS

VSS stands for "Volume Shadow Copy Service". Unlike virtual machine
snapshot, it only takes snapshot for the virtual disks, so both
filesystem and applications have to aware of it, and cooperate the
whole VSS process.

This driver exposes two device files to the userland:

/dev/hv_fsvss_dev

Normally userland programs should _not_ mess with this device file.
It is currently used by the hv_vss_daemon(8), which freezes and
thaws the filesystem. NOTE: currently only UFS is supported, if
the system mounts _any_ other filesystems, the hv_vss_daemon(8)
will veto the VSS process.

If hv_vss_daemon(8) was disabled, then this device file must be
opened, and proper ioctls must be issued to keep the VSS working.

/dev/hv_appvss_dev

Userland application can opened this device file to receive the
VSS freeze notification, hold the VSS for a while (mainly to flush
application data to filesystem), release the VSS process, and
receive the VSS thaw notification i.e. applications can run again.

The VSS will still work, even if this device file is not opened.
However, only filesystem consistency is promised, if this device
file is not opened or is not operated properly.

hv_vss_daemon(8) is started by devd(8) by default. It can be disabled
by editting /etc/devd/hyperv.conf.

Submitted by: Hongjiang Zhang <honzhan microsoft com>
Reviewed by: kib, mckusick
Sponsored by: Microsoft
Differential Revision: https://reviews.freebsd.org/D8224

308742
hyperv/vss: Nuke unused variables.

Submitted by: markj
Reported by: markj
Sponsored by: Microsoft

308743
hyperv/vss: Install the userland daemon to /usr/sbin instead of /

Submitted by: markj
Reported by: markj
Sponsored by: Microsoft

MFC r271493,271688-271689,271696,271854,272139-272143:

Import HyperV Key-Value Pair (KVP) driver and daemon code by Microsoft,
many thanks for their continued support of FreeBSD.

While I'm there, also implement a new build knob, WITHOUT_HYPERV to
disable building and installing of the HyperV utilities when necessary.

The HyperV utilities are only built for i386 and amd64 targets.

Approved by: re (gjb)

Import HyperV Key-Value Pair (KVP) driver and daemon code by Microsoft,
many thanks for their continued support of FreeBSD.

While I'm there, also implement a new build knob, WITHOUT_HYPERV to
disable building and installing of the HyperV utilities when necessary.

The HyperV utilities are only built for i386 and amd64 targets.

This is a stable/10 candidate for inclusion with 10.1-RELEASE.

Submitted by: Wei Hu <weh microsoft com>
MFC after: 1 week