discover/pxe-parser: Fix relative parsing for manual config files

Manually specified config files are asynchronously downloaded by
device_handler_process_url() before being parsed. This overwrites the
'pxeconffile' parameter, causing the parser to create relative paths
relative to the downloaded file's path, not the original remote path.

Work around this by setting 'pxeconffile-local' instead to differentiate
between the original config file's location and the local copy.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>