Change the location of the shared_info page for auto_translated_physmap guests.

Change the location of the shared_info page for auto_translated_physmap guests.
Instead of putting the page outside of the guests pseudo-physical address
space, we put it next to the other pages filled by the domain builder,
such that the page is already mapped in the initial pagetables and/or a
lowmem-type memory mapping.

Signed-off-by: Christian Limpach <Christian.Limpach@cl.cam.ac.uk>