Description of problem: The server-side XMLRPC handler for up2date, located at
/XMLRPC, needs to be able to handle having a Xen UUID in the hardware profile.

After looking through the code, it looks like the following modification will
have to be made:

In backend/server/rhnServer/server_hardware.py
     * Add XenDevice class that subclasses Device.
     * XenDevice class should accept receiving the 'class' and 'uuid' fields.
     * Once a table is defined for the Xen stuff, it's name will have to be
passed into the Device class constructor.
     * Change add_hardware so it knows about the "xen" class of hardware and
makes class_type and instance of XenDevice.
     * Change reload_hardware_byid so it will load info about the Xen
"hardware". May have to add a special case to __load_from_db().