[Users] Problem with time zone of a desktop VM

[Gianluca Cecchi]

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