hvm: Remove VM genearation ID device and incr_generationid from build_info.

hvm: Remove VM genearation ID device and incr_generationid from build_info.

Microsoft have now published their VM generation ID specification at
https://www.microsoft.com/en-us/download/details.aspx?id=30707.
It differs from the original specification upon which I based my
implementation in several key areas. Particularly, it is no longer
an incrementing 64-bit counter and so this patch is to remove
the incr_generationid field from the build_info and also disable the
ACPI device before 4.2 is released.

I will follow up with further patches to implement the VM generation
ID to the new specification.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Committed-by: Keir Fraser <keir@xen.org>