storage: unify permission formatting
    
    Volume and pool formatting functions took different approaches to
    unspecified uids/gids.  When unknown, it is always parsed as -1, but one
    of the functions formatted it as unsigned int (wrong) and one as
    int (better).  Due to that, our two of our XML files from tests cannot
    be parsed on 32-bit machines.
    
    RNG schema needs to be modified as well, but because both
    storagepool.rng and storagevol.rng need same schema for permission
    element, save some space by moving it to storagecommon.rng.
    
    Signed-off-by: Martin Kletzander <mkletzan@redhat.com>