ACPI: Add Virtual Machine Generation ID support

ACPI: Add Virtual Machine Generation ID support

This implements the VM Generation ID feature by passing a 128-bit
GUID to the guest via a fw_cfg blob.
Any time the GUID changes, an ACPI notify event is sent to the guest

The user interface is a simple device with one parameter:
 - guid (string, must be "auto" or in UUID format
   xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx)

Signed-off-by: Ben Warren <[email protected]>
Reviewed-by: Igor Mammedov <[email protected]>
Reviewed-by: Laszlo Ersek <[email protected]>
Tested-by: Laszlo Ersek <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>