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