virNetDevParseMcast: Avoid magic constant

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>