set custom loglevel for external libraries

In src/libxl/libxl_conf.c:libxlDriverConfigInit, virLogGetDefaultPriority is used to specify the (private) loglevel of an external library. This value could be controlled via "log_level=N" in libvirtd.conf. But doing it that way will affect libvirtd itself, instead of libxenlight.so as intended. There might be ways to suppress everything but libxl output via "log_filters=", but it is not clear what the syntax would be. I wonder what the preferred way is to specify the desired loglevel for the external library. It seems a separate configuration setting is required so that each value of XTL_* from xentoollog.h can be specified. Olaf