the logging configuration can be found / is documented in the INSTALL file shipped with the sources of each component (see below).
One important note as no used versions of OpenVAS/GVM 9 was provided yet:
Recently i have seen quite a lot user reporting issues but still running outdated versions of the OpenVAS/GVM components. Please compare your current used versions with the GVM-9 (stable, initial release 2017-03-07) ones to avoid that you’re having issues caused by such outdated versions.