docs: def _get_persistent_boot_devices(self):
          """Get details of persistent boot devices, its order
  
          :returns: List of dictionary of boot sources and
                    list of boot device order
          :raises: IloError, on an error from iLO.
          :raises: IloCommandNotSupportedError, if the command is not supported
                   on the server.
          """
          # Check if the BIOS resource if exists.
          headers_bios, bios_uri, bios_settings = self._check_bios_resource()
  
          # Get the Boot resource.
          boot_settings = self._get_bios_boot_resource(bios_settings)
  
          # Get the BootSources resource
          try:
              boot_sources = boot_settings['BootSources']
          except KeyError:
              msg = ("BootSources resource not found.")
              raise exception.IloError(msg)
  
          try:
              boot_order = boot_settings['PersistentBootConfigOrder']
          except KeyError:
              msg = ("PersistentBootConfigOrder resource not found.")
              raise exception.IloCommandNotSupportedError(msg)
  
          return boot_sources, boot_order