Several fixes to libvirtd's log setup

Several fixes to libvirtd's log setup

* qemud/qemud.c src/logging.[ch]: Similar as for general libvirt, don't
  convert high priority levels to debug level. Ignore LIBVIRT_LOG_FILTERS
  and LIBVIRT_LOG_OUTPUTS when they're set to the empty string, otherwise
  they can override a valid setting from the config file. Send all
  settings through the parser functions for validation, so that the
  existence of a bad setting doesn't nullify a good setting that should
  have applied -- particularly the default output. Keep the order of
  precedence consistent for all variables between the environment and
  the config file.  Warn when an invalid log level, filter, or output
  is ignored.
* src/libvirt_private.syms: export internally a few convenience functions