[libvirt] [PATCH 0/4] Updated patches 2-6 - virInterface test driver and netcf backend

These patches correspond to patches 2-6 in the set I sent yesterday (the 1st and 2nd had no changes, so they stand as-is). They incorporate all changes brought up in review of the first set. In particular: 1) When looking for a cached virInterface object for an interface, I now check that the mac address being searched for matches the one in the virInterface (case insensitive comparison). If it doesn't match, I create a new object rather than trying to modify the existing object with the same interface name, as that could have led to "improper behavior". 2) in virInterfaceFindByMACString() I now follow danpb's suggestion and use continue, rather than getting fancy with booleans. 3) testinterfaceCreate() and testInterfaceDestroy() now check if the interface has already been created/destroyed, and won't do the same operation twice in a row. 4) in the netcf backend driver, I now "roundtrip" the xml data in both interfaceDefineXML() and interfaceGetXMLDesc().