Disable receive ring slot accounting

Disable receive ring slot accounting

In the Netfront case it appears that by having scheduler decide when
to run the workload we don't need to involve additional mechanisms
to artificially limit resource availability to achieve better
performance under heavy load.  On the contrary, by performing a
single pass through Rx and Tx completion rings and having scheduler
decide when to run the next attempt, we limit the amount of time
spent in the packet processing and achieve system responsiveness.