Add preprocessor definitions for 32bit segment bases for use in
get_physical_address(). These will also be taken advantage of in the
next patch which adds KVM awareness.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>