This function is currently only called from one place, but in a
subsequent patch will be called from a 2nd place.

The new function exactly replicates the original behavior of the part
of virDomainActualNetDefFormat() that it replaces, but takes a
virDomainNetDefPtr instead of virDomainActualNetDefPtr, and uses the
virDomainNetGetActual*() functions whenever possible, rather than
reaching into def->data.network.actual - this is to be sure that we
are reporting exactly what is being used internally, just in case
there are any discrepancies (there shouldn't be).