Add Source to Resource.DiskInfo.

message Source {
      enum Type {
        PATH = 1;
        BLOCK = 2,
      }

      message Path {
        // Path to the folder (e.g., /mnt/raid/disk0).
        required string root = 1;
        required double total_size = 2;
      }

      message Block {
        // Path to the device file (e.g., /dev/sda1, /dev/vg/v1).
        // It can be a physical partition, or a logical volume (LVM).
        required string device = 1;
      }

      required Type type = 1;
      optional Path path = 2;
      optional Block block = 3;
    }
}