MAC address of a particular interface may change over time, and the
reduced virInterface object (which contains just name and mac) needs
to reflect these changes. Since we can't modify the mac address of an
existing virInterface (some other thread may currently be using it) we
just create a new virInterface, and let the old one die a dignified
death when its refct goes to 0.
* src/datatypes.c: fix the matching and lifetime of virInterface object
  accordingly