x86/xen: map foreign pfns for autotranslated guests

When running as a dom0 in PVH mode, foreign pfns that are accessed
must be added to our p2m which is managed by xen. This is done via
XENMEM_add_to_physmap_range hypercall. This is needed for toolstack
building guests and mapping guest memory, xentrace mapping xen pages,
etc.

Signed-off-by: Mukesh Rathor <mukesh.rathor@oracle.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>