Add support for virtual IPoIB interfaces (ipoib subinterface) & partition keys

Add support for virtual ipoib interfaces.  These are similar to ethernet macvlan.  It prevents the device being moved only into a single container.
Also added is support for Infiniband Partition Keys (which are similar to the ethernet VLAN).

Use like the following:

    pipework ib0 $CONTAINERID 10.10.10.1/24
    pipework ib0 $CONTAINERID 10.10.10.1/24 @8001  # for PKEY 8001 which must be set up in your subnet manager.

I've also updated the Readme for reflect the changes.
s/it's/its/