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

hyperv/vmbus: Declare Synic message and event w/ proper types

Avoid ugly casts.

MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D6446

hyperv/vmbus: Get rid of vmbus_devp

While I'm here, nuke useless print in vmbus_attach().

MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D6447