xen/page_alloc: Move get_pg_owner()/put_pg_owner() from x86 to common code

xen/page_alloc: Move get_pg_owner()/put_pg_owner() from x86 to common code

get_pg_owner() and put_pg_owner() will be necessary in a follow-up
commit to support xentrace on Arm. So move the helper to common code.

Take the opportunity to clean-up a bit the code moved.

Signed-off-by: Benjamin Sanda <ben.sanda@dornerworks.com>
[julien: Rework commit title / turn put_pg_owner to a static inline]
Signed-off-by: Julien Grall <julien.grall@arm.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>