Xen provides a UUID for running domains, but this UUID is only available from
dom0, not domU. Management applications (such as RHN) need a way to associate
the domain attached to a given dom0 with the running instance.
The method for paravirtualized hosts as discussed will probably be to expose the
UUID via /proc. Fully virtualized hosts will need to use a different method, so
another bug will be filed for them.
See bug #189498 for fully virtualized domains.
Looks like /sys/hypervisor/uuid exists in kernel-xen-2.6.17-1.2488.fc6