docs: def generate_disk_vdev(self, start_vdev=None, offset=0):
          """Generate virtual device number for disks
          :param offset: offset of user_root_vdev.
          :return: virtual device number, string of 4 bit hex.
          """
          if not start_vdev:
              start_vdev = CONF.zvm.user_root_vdev
          vdev = self._generate_vdev(start_vdev, offset)
          if offset >= 0 and offset < 254:
              return vdev
          else:
              msg = ("Failed to generate disk vdev, invalid virtual device"
                     "number for disk:%s" % vdev)
              LOG.error(msg)
              raise exception.SDKGuestOperationError(rs=2, msg=msg)