[HVM] Save/restore: clean up marshalling code

[HVM] Save/restore: clean up marshalling code
- All entries are now defined as structs and saved/restored
  in self-contained operations.
- Save/restore operations are type-safe, to tie each entry's
  typecode to a particular struct and its length.
- Save/restore handlers are registered once per host instead of
  per domain.
- Detect buffer overrun before it happens and abort.

Signed-off-by: Tim Deegan <Tim.Deegan@xensource.com>