CA-144882: don't complain about ENOENT when cleaning up the stats file

And ignore, but do log, errors when cleaning up the stats file.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Reviewed-by: Germano Percossi <germano.percossi@citrix.com>

GitHub: closes #123 on xapi-project/blktap