hyperv/vmbus: Add function to calculate max # of elements in a bufring.

Sponsored by:   Microsoft

hyperv/storvsc: Increase queue depth and rework channel selection.

- Increasing queue depth gives ~100% performance improvement for
  randwrite fio test in Azure.
- New channel selection, which takes LUN id and the current cpuid
  into consideration, gives additional ~20% performance improvement
  for ranwrite fio test in Azure.

Submitted by:   Hongzhang Jiang <honzhan microsoft com>
Modified by:    sephe
Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D7622