util: new functions virNetDev(Save|Read|Set)NetConfig()

util: new functions virNetDev(Save|Read|Set)NetConfig()

These three functions are destined to replace
virNetDev(Replace|Restore)NetConfig() and
virNetDev(Replace|Restore)MacAddress(), which both do the save and set
together as a single step. We need to separate the save, read, and set
steps because there will be situations where we need to do something
else in between (in particular, we will need to rebind a VF's driver
after save but before set).

This patch creates the new functions, but doesn't call them - that
will come in a subsequent patch. Note that the new functions to
read/write the file that stores the original network config now uses
JSON rather than plaintext (it still recognizes the old format as well
though, so it won't get confused during an upgrade).