vhost: revert avoid concurrency when logging dirty pages

vhost: revert avoid concurrency when logging dirty pages

This reverts commit 394313fff39d0f994325c47f7eab39daf5dc9e11.

While the patch did solve concurrency issue, it induces more
pages copies as some clean pages are marked as dirty for
performance reasons. Moreover, as there is no more contention
doing the logging, the rate of packets than can be processed is
higher, leading to even more pages to be dirtied.

It has been reported that with more than one queue pair, and
with a relatively low packet rate (1Mpps), the live migration
never converges until the flow is stopped.

While a better solution is found, it is better to reset to the
old behaviour, i.e. using atomic operation for dirty pages
logging.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>