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.