Tighten libvirt's parsing of logging env

Tighten libvirt's parsing of logging env

* src/libvirt.c src/logging.c: Don't convert high priority levels to the
  debug level. Don't parse LIBVIRT_LOG_FILTERS and LIBVIRT_LOG_OUTPUTS
  when they're set to the empty string. Warn when the user specifies an
  invalid value (empty string remains a noop).
* po/POTFILES.in: src/logging.c now include translatable strings