x86: correct boot time page table setup

x86: correct boot time page table setup

While using alloc_domheap_pages() and assuming the allocated memory is
directly accessible is okay at boot time (as we run on the idle page
tables there), memory hotplug code too assumes it can access the
resulting page tables without using map_domain_page() or alike, and
hence we need to obtain memory suitable for ordinary page table use
here.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Julien Grall <julien.grall@arm.com>