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
MFC after: 1 week
Sponsored by: Microsoft
Differential Revision: https://reviews.freebsd.org/D7622