hvmloader: Allocate an 8 byte buffer to contain the VM generation id

hvmloader: Allocate an 8 byte buffer to contain the VM generation id
and populate it at boot time with a value read from
"platform/generation_id". Also add code to libxl to populate this
xenstore key with the value of a new 'generation_id' parameter in the
VM config file.  Populate the ADDR package of VM_Gen_Counter ACPI
device such that the first integer evaluates to the low order 32 bits
of the buffer address and the second integer evaluates to the high
order 32 bits of the buffer address.

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