How to reserve guest physical region for ACPI

[Xiao Guangrong <guangrong.xiao@linux.intel.com>]

Hi Michael, Paolo,

Now it is the time to return to the challenge that how to reserve guest
physical region internally used by ACPI.

Igor suggested that:
| An alternative place to allocate reserve from could be high memory.
| For pc we have "reserved-memory-end" which currently makes sure
| that hotpluggable memory range isn't used by firmware
(https://lists.nongnu.org/archive/html/qemu-devel/2015-11/msg00926.html)

he also innovated a way to use 64-bit address in DSDT/SSDT.rev = 1:
| when writing ASL one shall make sure that only XP supported
| features are in global scope, which is evaluated when tables
| are loaded and features of rev2 and higher are inside methods.
| That way XP doesn't crash as far as it doesn't evaluate unsupported
| opcode and one can guard those opcodes checking _REV object if neccesary.
(https://lists.nongnu.org/archive/html/qemu-devel/2015-11/msg01010.html)

Michael, Paolo, what do you think about these ideas?

Thanks!