hyperv/stor: Set description properly in probe devmethod

MFC after:  1 week
Sponsored by:       Microsoft OSTC

hyperv/hn: Change description to "Hyper-V Network Interface"

This is consistent w/ other Hyper-V devices.

MFC after:  1 week
Sponsored by:       Microsoft OSTC

hyperv/hn: Restart sending earlier once we gathered some free TX descs

This greatly reduces the oqdrops under heavy workload.

For TCP send/recv test (10K concurrent connections):
oqdrops is reduced by 17% on sending side, and 57% on receiving side.

For nginx-1.8/wrk-4 1KB object test (10K concurrent connections,
4 requests/connection):
oqdrops is reduced by 44% on nginx side, and 10% on wrk side.

MFC after:  1 week
Sponsored by:       Microsoft OSTC

hyperv/hn: Remove unapplied comment.

Chimney sending buffers are shared across channels.

MFC after:  1 week
Sponsored by:       Microsoft OSTC