Merge branch 'CA-134745' into xs64bit

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>

GitHub: closes #85 on xapi-project/blktap
CA-135969: disable verbose messages in tapback

They seem to significantly delay tapback's execution.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
CA-135969: don't log to separate file, use syslog instead

Not logging to syslog leads to nasty lock ups and long delays.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>