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.
          :raises: IloCommandNotSupportedError, if the command is not supported
                   on the server.
          """
          system = self._get_host_details()
          try:
              # Return boot device if it is persistent.
              if system['Boot']['BootSourceOverrideEnabled'] == 'Continuous':
                  device = system['Boot']['BootSourceOverrideTarget']
                  if device in DEVICE_RIS_TO_COMMON:
                      return DEVICE_RIS_TO_COMMON[device]
                  return device
          except KeyError as e:
              msg = "get_persistent_boot_device failed with the KeyError:%s"
              raise exception.IloError((msg) % e)
  
          # 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
  
          # Get persistent boot device order for UEFI
          boot_sources, boot_devices = self._get_persistent_boot_devices()
  
          boot_string = ""
          try:
              for source in boot_sources:
                  if (source["StructuredBootString"] == boot_devices[0]):
                      boot_string = source["BootString"]
                      break
          except KeyError as e:
              msg = "get_persistent_boot_device failed with the KeyError:%s"
              raise exception.IloError((msg) % e)
  
          if 'HP iLO Virtual USB CD' in boot_string:
              return 'CDROM'
  
          elif ('NIC' in boot_string or
                'PXE' in boot_string or
                "iSCSI" in boot_string):
              return 'NETWORK'
  
          elif common.isDisk(boot_string):
              return 'HDD'
  
          else:
              return None