[PATCH] virtio: Avoid virtio_net TX queue over run

[Shirley Ma <mashirle@us.ibm.com>]

This patch addresses small message size performance in a situation the
KVM guest virtio_net TX queue overrun. This patch adds a new API in
virtio_ring for ring capacity check; and remove KVM guest virtio_net TX
queue send completion interrupts completely. The test has shown that 
whenever the queue is overrun, it's much better to drop a few packets
than stopping TX queue and waiting for host to notify the guest to wake
up the TX queue again, the small messages size performance gain for
single TCP_STREAM BW could be up to 200%-300% and better than bare
metal, and no regression has been found in other situation.

Performance data for 10GbE,

KVM guest to local host:
------------------------
Message size	2.6.38-rc8 	2.6.38-rc8+patch
1024		1770.61		4528.37
2048		2702.30		7110.95
4096		5256.84		10104.76
8192		7543.66		10945.93
16K		10500.47	10783.50
64K		13718.62	13640.80

KVM guest to remote host:
--------------------------
Message size	Bare Metal  2.6.38-rc8	2.6.38-rc8+patch
1024		1802.67		2381.41		5599.15	
2048		4317.87		4094.12		9241.86
4096		6266.15		5231.24		9321.87
8192		8409.17		7952.74		9265.45
16K		9351.63		8260.68		8310.29
64K		9347.94		9103.75		9094.38 

Thanks
Shirley