CA-137237 Fixed XenCenter fatal exception when rebuilding alertList

- HelpersGUI.DateTimeToString() needs to be called on the event thread.

Signed-off-by: Mihaela Stoica <mihaela.stoica@citrix.com>
Merge pull request #88 from MihaelaStoica/CA-137237

Fixed XenCenter fatal exception when rebuilding alertList