xen/arm: Make get_page_from_gfn working with DOMID_XEN

xen/arm: Make get_page_from_gfn working with DOMID_XEN

DOMID_XEN is used to share pages beloging to the hypervisor
(e.g trace buffers). Unlike other domains, DOMID_XEN is a non-auto
translated domain and therefore does not have a P2M.

This patch adds a special case for DOMID_XEN in get_page_from_gfn. We
may want to provide "non-auto translated helpers" in the future if we
see more case.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Andrii Anisov <andrii_anisov@epam.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>