Correct the ID String of Disk Size in Template Edit Window

It was "version" instead of "disk".

v2 -> v3:
  3a) Corrected id string from CDROM
      (Aline's comment)

v1 -> v2:
  2a) Added for attribute for version text box
      (Sheldon's comment)

Signed-off-by: Hongliang Wang <hlwang@linux.vnet.ibm.com>