vhost: cache used event for better performance

When event index was enabled, we need to fetch used event from
userspace memory each time. This userspace fetch (with memory
barrier) could be saved sometime when 1) caching used event and 2)
if used event is ahead of new and old to new updating does not cross
it, we're sure there's no need to notify guest.

This will be useful for heavy tx load e.g guest pktgen test with Linux
driver shows ~3.5% improvement.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>