engine: can not change template vm type

Edit template->change vm type from server to desktop (or opposite).
It failed on: VMT_CANNOT_UPDATE_ILLEGAL_FIELD

Fixed by completing this field to the VmTemplateHandler.

Change-Id: Ie1267726ff7d57351caedd773542cba65e0eca1d
Bug-Url: https://bugzilla.redhat.com/1000964
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>