webadmin: disable format selection on thin provisioning

VM dialog -> resource allocation:
disk format should be unchangeable when storage
thin provisioning is selected (changing format is
applicable only on clone template provisioning).

Change-Id: I6b270bb753bcceb4c085db79e413cf9ed3bc04e3
Bug-Url: https://bugzilla.redhat.com/1101003
Signed-off-by: Daniel Erez <derez@redhat.com>