vhost: Improve data path

This patch significantly improves vhost Tx and Rx
performance (20% Mpps improvement in PVP scenario).

Most significant change consists in the split of
virtio ring parsing and memory copies.
Other minor changes mostly consist in getting
rid of unnecessary variables.

Change-Id: Ia94e12ab9a6c114e3f7fef376a9343823d051209
Signed-off-by: Pierre Pfister <ppfister@cisco.com>