hyperv/channel: Add debug sysctl nodes for channel indices

It would serve as a debug tool, if the shared buffer ring's indices
stopped updating.

Submitted by: HongJiang Zhang <honzhan microsoft com>
Reviewed by: sephe, Jun Su <junsu microsoft com>
Modified by: sephe
MFC after: 1 week
Sponsored by: Microsoft OSTC
Differential Revision: https://reviews.freebsd.org/D5402