These new functions are highly inspired by those in domain_conf.c (but
not identical), and are intended to make it simpler to update the
various combinations of live/persistent network configs.

The network driver wasn't previously as careful about the separation
between the live "status" in network->def and the persistent "config"
in network->newDef (or sometimes in network->def). This series
attempts to remedy some of that, but probably doesn't go all the way
(enough to get these functions working and enable continued work on
virNetworkUpdate though).

bridge_driver.c and test_driver.c were updated in a few places to take
advantage of the new functions and/or account for changes in argument
lists.