hyperv/utils: Code rearrange and cleanup

Split heartbeat, shutdown and timesync out of utils code
and name them properly.

Submitted by: Jun Su <junsu microsoft com>
Reviewed by: adrian, sephe, Hongjiang Zhang <honzhan microsoft com>
MFC after: 1 week
Sponsored by: Microsoft OSTC
Differential Revision: https://reviews.freebsd.org/D5216