hvm: infrastructure for backwards-compatible loading

hvm: infrastructure for backwards-compatible loading

The hvm_save code is used to save and restore hypervisor-related
hvm state, either for classic save/restore, or for migration
(including remus).  This is meant to be backwards-compatible across
some hypervisor versions; but if it does change, there is no way to
handle the old format as well as the new.

This patch introduces the infrastructure to allow a single older
version ("compat") of any given "save type" to be defined, along with
a function to turn the "old" version into the "new" version.  If the
size check fails for the "normal" version, it will check the "compat"
version, and if it matches, will read the old entry and call the
conversion function.

This patch involves some preprocessor hackery, but I'm only extending the
hackery that's already there.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Tim Deegan <Tim.Deegan@citrix.com>
Committed-by: Tim Deegan <Tim.Deegan@citrix.com>