Since I can’t really make head or tails of the xend/xenstore and the virsh way of configuring domains to autostart, I’ve just settled with exporting the config files from virsh and converting them to the native xen format.
virsh dumpxml hydrogen > /etc/xen/hydrogen.xml && virsh domxml-to-native /etc/xen/hydrogen.xml > /etc/xen/hydrogen && ln -s /etc/xen/hydrogen /etc/xen/auto/hydrogen
(Ok, the last step was by memory. But the syntax should be correct…)
Edit @ 30th Apr 2011: Apparently the syntax has changed. Converting from xml to native is now
virsh domxml-to-native xen-xm /etc/xen/hydrogen.xml > /etc/xen/hydrogen