fix XENMEM_add_to_physmap_range preemption handling

fix XENMEM_add_to_physmap_range preemption handling

Just like for all other hypercalls we shouldn't be modifying the input
structure - all of the fields are, even if not explicitly documented,
just inputs (the one OUT one really refers to the memory pointed to by
that handle rather than the handle itself).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Tim Deegan <tim@xen.org>
Acked-by: Keir Fraser <keir@xen.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>