conf: clarify what is returned for actual bandwidth and vlan

conf: clarify what is returned for actual bandwidth and vlan

In practice, if a virDomainNetDef has a virDomainActualNetDef
allocated, the ActualNetDef will *always* contain the bandwidth and
vlan data from the NetDef (unless there was also a portgroup involved
- see networkAllocateActualDevice()).

However, virDomainNetGetActual(Bandwidth|Vlan)() were coded to make it
appear as if it might be possible to have a valid bandwidth/vlan in
the NetDef, but a NULL in the ActualNetDef. Believing this un-truth
could lead to writing unnecessarily defensive code when dealing with
the virDomainGetActual*() functions, so this patch makes it more
obvious:

   If there is an ActualNetDef, it will always have a copy of the
   various appropriate bits from its parent NetDef, and the
   virDomainGetActual* function will *always* return the data from the
   ActualNetDef, not from the NetDef.

The reason for this effective-NOP patch is that a subsequent patch to
change virDomainNetDefFormat will rely on the above rule.