xen: Kconfig: nest Xen guest options

xen: Kconfig: nest Xen guest options

Moving XEN_512GB allows it to nest under XEN_PV.  That also allows
XEN_PVH to nest under XEN as a sibling to XEN_PV and XEN_PVHVM giving:

[*]   Xen guest support
[*]     Xen PV guest support
[*]       Limit Xen pv-domain memory to 512GB
[*]       Xen PV Dom0 support
[*]     Xen PVHVM guest support
[*]     Xen PVH guest support

Signed-off-by: Jason Andryuk <[email protected]>
Reviewed-by: Juergen Gross <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Juergen Gross <[email protected]>