XEN_BACKEND doesn't actually depend on XEN_DOM0. DomUs can serve
backends to other DomUs. One example is a service VM providing network
backends.
The original Kconfig defaulted Dom0 to y and it could be disabled. DomU
could not select the option. With the new Kconfig, we default y for
Dom0 and n for DomU. Either can then toggle the selection.
Signed-off-by: Jason Andryuk <jandryuk@gmail.com> Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com> Signed-off-by: Juergen Gross <jgross@suse.com>