[libvirt] Clock problems with Windows guests and Xen

When I start Windows guests with libvirt my clocks are 5 hours off. When I use a Xen configuration file, I can specify an offset (rtc_timeoffset = -18000) to get my clock to sync with the host. Is there a way to do specify this offset in XML with libvirt? Right now, I'm using version 0.4.4 but I downloaded the source for 0.5.1 and it looks like that part of domain_conf.c hasn't changed. Thanks for any help! -matthew