When we read a lxc.network.hwaddr line, if it contained any 'x's then
those get quitely filled in at config_network_hwaddr. If that happens
then we want to save the autogenerated hwaddr in the unexpanded config
so that when we write it to disk, it is saved.
This patch dumbly re-generates the network configuration in the
unexp configuration every time we load a config file, just as we do
after every clone.