This bug was revealed by the crash described in

  https://bugzilla.redhat.com/show_bug.cgi?id=852383

The vlan info pointer sent to virNetDevOpenvswitchAddPort should never
be non-NULL unless there is at least one tag. The factthat such a vlan
info pointer was receveid pointed out that a caller was passing the
wrong pointer. Instead of sending &net->vlan, the result of
virDomainNetGetActualVlan(net) should be sent - that function will
look for vlan info in net->data.network.actual->vlan, and in cany case
return NULL instead of a pointer if the vlan info it finds has no
tags.

Aside from causing the crash, sending a hardcoded &net->vlan has the
effect of ignoring vlan info from a <network> or <portgroup> config.