x86/mm: Consolidate all Xen L2 slot writing into init_xen_pae_l2_slots()

x86/mm: Consolidate all Xen L2 slot writing into init_xen_pae_l2_slots()

Having all of this logic together makes it easier to follow Xen's virtual
setup across the whole system.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Tim Deegan <tim@xen.org>
Release-acked-by: Julien Grall <julien.gral@linaro.org>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>