Revert "x86/mm: move PV l4 table setup code" and "x86/mm: factor out pv_arch_init_memory"

Revert "x86/mm: move PV l4 table setup code" and "x86/mm: factor out pv_arch_init_memory"

This reverts commit f3b95fd07fdb55b1db091fede1b9a7c71f1eaa1b and
1bd39738a5a34f529a610fb275cc83ee5ac7547a.

The following patches (post XSA-243 fixes) requires init_guest_l4_table()
being common code.

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