vhost: arrange struct fields for better cache sharing

vhost: arrange struct fields for better cache sharing

The ifname[] field takes so much space, that it seperates some frequently
used fields into different caches, say, features and broadcast_rarp.

This patch moves all those fields that will be accessed frequently in Rx/Tx
together (before the ifname[] field) to let them share one cache line.

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