Use a thread for the processing of virtio tx descriptors rather
than blocking the vCPU thread. This improves bulk data performance
by ~30-40% and doesn't harm req/resp time for stock netperf runs.

Future work will use a thread pool rather than a thread per tx queue.

Submitted by:	Dinakar Medavaram
Reviewed by:	neel, grehan
Obtained from:	NetApp
Remove a comment that shouldn't have gone in.

X-MFC-with:	r249864