docs: def image_get_root_disk_size(self, image_name):
          """Return the root disk units of the specified image
          image_name: the unique image name in db
          Return the disk units in format like 3339:CYL or 467200:BLK
          """
          image_info = self.image_query(image_name)
          if not image_info:
              raise exception.SDKImageOperationError(rs=20, img=image_name)
          disk_size_units = image_info[0]['disk_size_units'].split(':')[0]
          return disk_size_units