hvm: add HVM_PARAM_VM_GENERATION_ID_ADDR

hvm: add HVM_PARAM_VM_GENERATION_ID_ADDR

HVM_PARAM_VM_GENERATION_ID_ADDR is the guest physical address of the
VM Generation ID.  This parameter will be written by hvmloader and read
by the toolstack when updating the gen. ID (e.g., after restoring from a
snapshot).

A HVM parameter is easier for the save/restore code to work with (than
a XenStore key).

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>