Add rudimentary ACPI support which allows a fully virtualized domain

Add rudimentary ACPI support which allows a fully virtualized domain
to perform a poweroff via the S5 state.  This involves adding S5
information to the ASL file; adding ACPI I/O address processing to
qemu and modifying the FADT to point to these specific I/O addresses.
Note that this is a rudimentary implementation, which is expected
to allow growth over time.  Currently, only allow/detect writes
to one of the PM registers, even though the S5 package specifies
data for both.  Currently, this is sufficient for the kernels
tested. (Specifically, use of _PST, and _GST and other methods
are unnecessary and not implemented.)

Signed-off-by: Ben Thomas <ben@virtualiron.com>