conf: improve the way we format blkiotune and cputune
    
    Just refactor existing code to use a child buf instead of
    check all element before format <blkiotune> and <cputune>.
    This will avoid the more and more bigger element check during
    we introduce new elements in <blkiotune> and <cputune> in the
    future.
    
    Signed-off-by: Luyao Huang <lhuang@redhat.com>