[Users] Problem with time zone of a desktop VM

Hello, oVirt 3.2.1 on Fedora 18 all-in-one configuration. I created a Windows XP with wrong time zone (default that is UTC -12 .. why? Don't reply why not, please... ;-) Now if the VM is powered off and I edit it, I can't still change its timezone, because it is greyed out in initial run section. Tried to change inside guest but at next reboot it keeps with same problem. How can I change it after creation? How to change default so that it is not UTC-12 but matches my timezone for every desktop I create? Thanks, Gianluca