MFC 305111

MFC 305111

    hyperv/timesync: Rework time adjustment policy

    - By default, adjust time upon SYNC request.  It can be disabled
      through hw.hvtimesync.ignore_sync_req.  SYNC request will be
      sent by hypervisor the host is resumed, rebooted, etc.
    - By default, adjust time upon SAMPLE request, if there is 100ms
      difference between VM time and hypervisor time.  This can be
      disabled through hw.hvtimesync.sample_drift.

    And nuke the unnecessary task, since channel callback is running
    in a Hyper-V taskqueue nowadays.

    Submitted by:   YanZhe Chen <t-yachen microsoft com>
    Discussed with: Dexuan Cui <decui microsoft com>, Hongjiang Zhang <honzhan microsoft com>, sephe
    Sponsored by:   Microsoft
    Differential Revision:  https://reviews.freebsd.org/D7707

git-svn-id: svn://svn.freebsd.org/base/stable/10@307187 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f