bus: Fix maximum log levels when mixing log/vlog implementing loggers

bus: Fix maximum log levels when mixing log/vlog implementing loggers

The maximum would not get set correctly when a logger is removed and the
first remaining logger in the list (the one with the highest log level) does
e.g. only implement vlog() while there are other loggers that implement log().
This would result in only max_vlevel getting set correctly while max_level
would incorrectly get set to -1 so that log() would not get called for any
of the loggers anymore.

References #574.