xen/page_alloc: statically allocate bootmem_region_list

xen/page_alloc: statically allocate bootmem_region_list

The existing code assumes that the first mfn passed to the boot
allocator is mapped, which creates problems when, e.g., we do not have
a direct map, and may create other bootstrapping problems in the
future. Make it static. The size is kept the same as before (1 page).

Signed-off-by: Hongyan Xia <hongyxia@amazon.com>
Reviewed-by: Julien Grall <julien@xen.org>
Reviewed-by: Jan Beulich <jbeulich@suse.com>