xen: Make XEN_BACKEND selectable by DomU

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>