hyperv/et: Fix STIMER0 operations.

- Make sure that STIMER0 is disabled before writting to it, since
  writing to an enabled STIMER will result in undefined behaviour.
- It is unnecessary to reconfigure STIMER0 upon each et_start().
- Make sure that MSR_HV_REF_TIME_COUNT will not return 0, since
  writing 0 to STIMER_COUNT will disable the target STIMER.

MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D6573

hyperv/vmbus: Move SINT settings to vmbus_var.h

While I'm here remove the event timer's dependency on hv_vmbus_priv.h

MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D6574

hyperv/et: Make sure only one event timer will be registered

This nullifies the need to use softc.

MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D6591

hyperv: Move timer frequency definition to common place.

And cleanup event timer period settings.

MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D6597

hyperv/et: Device renaming; consistent w/ other Hyper-V utils

While I'm here, prefix function names w/ vmbus, since unlike Hyper-V
timecounter, Hyper-V event timer will not work w/o vmbus.

MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D6598

hyperv/et: Allow Hyper-V event timer be disabled

MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D6599

hyperv/vmbus: Process event timer before checking events

And update comment.

MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D6600