hvm: Fix ACPI shutdown, broken by my previous changeset.

hvm: Fix ACPI shutdown, broken by my previous changeset.

It turns out that although PIIX4 hardware defines the S5 type code to
be 000, all OSes will discover the correct code by evlauating an \_Sx
object in the ACPI DSDT. And we set the type code in that object to be
111.

So this patch keeps the other cleanups made to the piix4acpi.c file,
but switches back to checking for code 111. It also makes it clearer
in both the ioemu code and in the dsdt source code where these magic
numbers come from.

Let's hope noone actually has the true PIIX4 type codes hardcoded
(it's highly doubtful that anyone would).

Signed-off-by: Keir Fraser <keir@xensource.com>