MFC 304790,304791

MFC 304790,304791

304790
    hyperv/vmbus: Add function to calculate max # of elements in a bufring.

    Sponsored by:   Microsoft

304791
    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

git-svn-id: svn://svn.freebsd.org/base/stable/10@307180 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f