Create a qdev plugged to the xen-sysbus for each new backend device.
This device can be used as a parent for all needed devices of that
backend. The id of the new device will be "xen-<type>-<dev>" with
<type> being the xen backend type (e.g. "qdisk") and <dev> the xen
backend number of the type under which it is to be found in xenstore.