virNetDevParseMcast: Avoid magic constant
    
    There is no guarantee that an enum start it mapped onto a value
    of zero. However, we are guaranteed that enum items are
    consecutive integers. Moreover, it's a pity to define an enum to
    avoid using magical constants but then using them anyway.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>