x86/ACPI: invalidate BGRT if necessary

x86/ACPI: invalidate BGRT if necessary

Since the image pointed to may live in boot services memory (which we
add to the global memory pool long before ACPI tables get looked at),
we should prevent Dom0 from trying to retrieve the image data in that
case.

The alternatives would be to
- not add boot services memory to the global pool at all, or
- defer adding boot services memory until Dom0 indicates it is safe to
  do so, or
- find and parse the BGRT table in xen/arch/x86/efi/boot.c, and avoid
  adding that specific region to the E820 table.
None of these are really attractive, and as Xen commonly prints to the
video console anyway (without trying to avoid any regions on the
screen), the invalidation would need to be done conditionally anyway.

(xen/include/acpi/actbl3.h is a verbatim copy from Linux 3.7-rc4)

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>