Low: libpe_status: add sanity check when unpacking migration history

e89ac6d (1.1.4) bumped the feature set when it changed the recording and
processing of migrate_to and migrate_from action history. That would ensure
live clusters wouldn't have any problems with the changes.

However it didn't take any precautions for processing old saved CIB files.
These would either work by coincidence or fail. We don't support such old files
in Pacemaker 2, so just add a sanity check to ensure we have the right
information.