[VTPM] Make vTPM hotplug scripts use the UUID of the vTPM instance

[VTPM] Make vTPM hotplug scripts use the UUID of the vTPM instance

Make the vTPM hotplug script use the UUID of the vTPM instance for
associating it with its instance number rather than the name of the
domain.
Also change the cleanup in the xm test to use the atexit mechanism
rather than explicitly calling a function to clean up.

Signed-off-by: Stefan Berger <stefanb@us.ibm.com>