hyperv: Move Hypercall setup to an early place.
It does not belong to the vmbus.
While I'm here rework the Hypercall setup, e.g. use busdma(9)
and avoid bit fields.
Discussed with: Jun Su <junsu microsoft com>
MFC after: 1 week
Sponsored by: Microsoft OSTC
Differential Revision: https://reviews.freebsd.org/D6445