cpu: Guard cpu_{save,load}() definitions

A few targets already managed to implement cpu_save() and cpu_load()
without defining CPU_SAVE_VERSION that causes them to be registered.

Guard the prototypes with CPU_SAVE_VERSION to avoid this happening again
until all targets are converted to VMState (or QIDL).

Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
