vhost: batch used descs chains write-back with packed ring

vhost: batch used descs chains write-back with packed ring

Instead of writing back descriptors chains in order, let's
write the first chain flags last in order to improve batching.

Also, move the write barrier in logging cache sync, so that it
is done only when logging is enabled. It means there is now
one more barrier for split ring when logging is enabled.

With Kernel's pktgen benchmark, ~3% performance gain is measured.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Tiwei Bie <tiwei.bie@intel.com>