xenserver: Rework interface-reconfigure.

xenserver: Rework interface-reconfigure.

Substantially reworks interface-reconfigure, with the following fixes:

      * Create and use ifcfg files only for ipdev, use vswitch
        configuration for topology setup.

      * Take care over moving from bond to slave and back to tear down
        any residual sibling devices

      * Take care to leave datapath present when manipulating VLANs to
        avoid interrupting traffic on the slave PIF as well as other
        VLANs.

      * Lots of minor stuff