Low: log: return filter type enum to preserve abi compatibilty with corosync

This returns abi compatiblity with libqb v0.16.0. There was a subtle
change in behavior caused by reordering the log filter enum in
v0.17.0.
Merge pull request #127 from davidvossel/17.1.fixes

Low: log: return filter type enum to preserve abi compatibilty with corosync