libxl: allow a generation ID to be specified at domain creation

libxl: allow a generation ID to be specified at domain creation

Toolstacks may specify a VM generation ID using the u.hvm.ms_vm_genid
field in the libxl_domain_build_info structure, when creating a
domain.

The toolstack is responsible for providing the correct generation ID
according to the Microsoft specification (e.g., generating new random
ones with libxl_ms_vm_genid_generate() as appropriate when restoring).

Although the specification requires that a ACPI Notify event is raised
if the generation ID is changed, the generation ID is never changed
when the domain is in a state to receive such an event (it's either
newly created or suspended).

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Cc: David Scott <dave.scott@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>