virtio: optimize avail ring update

virtio: optimize avail ring update

Avail ring is updated by the frontend and consumed by the backend.
There are frequent core to core cache transfers for the avail ring.

This optmization avoids avail ring entry index update if the entry
already holds the same value.
As DPDK virtio PMD implements FIFO free descriptor list (also for
performance reason of CACHE), in which descriptors are allocated
from the head and freed to the tail, with this patch in most cases
avail ring will remain the same, then it would be valid in both caches
of frontend and backend.

Suggested-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Huawei Xie <huawei.xie@intel.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>