docs: def get_persistent_boot_device(self):
          """Get current persistent boot device set for the host
  
          :returns: persistent boot device for the system
          :raises: IloError, on an error from iLO.
          """
          sushy_system = self._get_sushy_system(PROLIANT_SYSTEM_ID)
          # Return boot device if it is persistent.
          if ((sushy_system.
               boot.enabled) == sushy.BOOT_SOURCE_ENABLED_CONTINUOUS):
              return PERSISTENT_BOOT_MAP.get(sushy_system.boot.target)
          # Check if we are in BIOS boot mode.
          # There is no resource to fetch boot device order for BIOS boot mode
          if not self._is_boot_mode_uefi():
              return None
  
          try:
              boot_device = (sushy_system.bios_settings.boot_settings.
                             get_persistent_boot_device())
              return PERSISTENT_BOOT_MAP.get(boot_device)
          except sushy.exceptions.SushyError as e:
              msg = (self._("The Redfish controller is unable to get "
                            "persistent boot device. Error %(error)s") %
                     {'error': str(e)})
              LOG.debug(msg)
              raise exception.IloError(msg)