migration: Report values for comparisons

Report the values when a comparison fails; together with
the previous patch that prints the device and field names
this should give a good idea of why loading the migration failed.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>