The main thrust of this is to not create virtual interfaces until
applying the config.
This meant that the network model has to change a bit to be able to
represent interfaces that do not yet exist on the system. I did this
by ripping out most of the existing network device code: now a
NetworkDev is really just a wrapper for the config for a device and (if
it exists) the netlink data too. A few places had to adjust to checking
if the netlink info is available before accessing it but all in all it
was not that painful.
There are a few other refactorings in this commit that perhaps should be
split out (how the bond parameters are handled, some stuff about
resizing the table rows when interfaces are edited) but it doesn't
really seem worth it.
15b6bb1114