x86/mm: switch to new APIs in arch_init_memory

x86/mm: switch to new APIs in arch_init_memory

The function will map and unmap pages on demand.

Since we now map and unmap Xen PTE pages, we would like to track the
lifetime of mappings so that 1) we do not dereference memory through a
variable after it is unmapped, 2) we do not unmap more than once.
Therefore, we introduce the UNMAP_DOMAIN_PAGE macro to nullify the
variable after unmapping, and ignore NULL.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Hongyan Xia <hongyxia@amazon.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>