This is the one function outside of domain_conf.c that plays around
with (even modifying) the internals of the virDomainNetDef, and thus
can't be fixed up simply by replacing direct accesses to the fields of
the struct with the GetActual*() access functions.

In this case, we need to check if the defined type is "network", and
if it is *then* check the actual type; if the actual type is "bridge",
then we can at least put the bridgename in a place where it can be
used; otherwise (if type isn't "bridge"), we behave exactly as we used
to - just null out *everything*.