Re: [libvirt] [PATCH] logging: remove concept of default log priority

The logging framework has categories which can be selectively enabled/disabled by setting a suitable LIBVIRT_LOG_FILTERS environment variable or config file setting. Along side that we also have the LIBVIRT_DEBUG environment variable which unconditionally enables every single category at DEBUG level. With the amount of logging produced by libvirt these days, the signal/noise ratio in the output from setting LIBVIRT_DEBUG is to poor for it to be useful. Originally the LIBVIRT_DEBUG env variable had a very specific use case - it resulted in logging of anything in our public API entrypoints. eg it turned on src/libvirt.c debugging and nothing else. Additionally it would always result in log messages being sent to stderr. When applied to any of the daemons, the log output no longers gets sent to stderr, but rather to whatever default log output has been configured by the daemon. If no log_outputs setting or LIBVIRT_LOG_OUTPUTS env is set, then messages will often go to the systemd journal or a /var/log/libvirt/libvirtd.log file rather than stderr. These factors have conspired to make the LIBVIRT_DEBUG env and/or default log priority to be pretty useless in the real world. This change attempts to take us back towards the original semantics of the LIBVIRT_DEBUG env variable as follows. If LIBVIRT_DEBUG is set to a plain integer, or log level string, then it will turn on logging for the "libvirt" log category at that level. Any other string will be parsed in the same way as LIBVIRT_LOG_FILTERS would be. In all cases use of LIBVIRT_DEBUG will result in an explicit output being added for stderr. This ensures that messages always go to stderr, even if other outputs are already configured. IOW, LIBVIRT_DEBUG=1 virsh or LIBVIRT_DEBUG=1 libvirtd will both result in printing logs of libvirt public API calls to stderr. Meanwhile setting LIBVIRT_DEBUG="1:qemu" is equivalent to setting LIBVIRT_LOG_FILTERS="1:qemu" and LIBVIRT_LOG_OUTPUTS="1:stderr"