Internal variant of freecpu taking a libvirt connection as parameter

def _freecpu(conn):
    '''
    Internal variant of freecpu taking a libvirt connection as parameter
    '''
    cpus = conn.getInfo()[2]
    for dom in _get_domain(conn, iterable=True):
        if dom.ID() > 0:
            cpus -= dom.info()[3]
    return cpus