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