vhost: pre update used ring for Tx and Rx

vhost: pre update used ring for Tx and Rx

Pre update and update used ring in batch for Tx and Rx at the stage
while fetching all avail desc idx. This would reduce some cache misses
and hence, increase the performance a bit.

Pre update would be feasible as guest driver will not start processing
those entries as far as we don't update "used->idx". (I'm not 100%
certain I don't miss anything, though).

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Tested-by: Rich Lane <rich.lane@bigswitch.com>