format: make swallow method pay a bigger attention to validations
    
    Previously, it would only care if the representation of the format
    instance to be stored was validatable.  This neglience was circumventing
    the idea of "what's been constructed successfully is assuredly valid".
    
    It becomes even more pressing because we want to introduce the concept
    of "one-way upgradability of the format" (treating multiple similar
    versions in the same family of formats uniformly, with implicit
    on-demand upgrades where required), and it's not reasonably doable
    in case of CIB without keeping tabs on the validity properly
    (as the lack thereof is an impulse to try upgrading, similarly
    to what pacemaker does under the hood).
    
    Signed-off-by: Jan Pokorný <jpokorny@redhat.com>