xen/setup: Don't relocate p2m over existing one

When relocating the p2m, take special care not to relocate it so
that is overlaps with the current location of the p2m/initrd. This is
needed since the full extent of the current location is not marked as a
reserved region in the e820.

This was seen to happen to a dom0 with a large initial p2m and a small
reserved region in the middle of the initial p2m.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>