support QoS in OVS within Mininet's TCLinks

introduces two new switch classes: OVSHtbQosSwitch and OVSHfscQosSwitch
which allow one to use Open vSwitch's HTB or HFSC QoS support as well
as Mininet's TCLink / TCIntf classes. By using one of these two switch
classes, a Linux tc hierarchy is pre-created for Open vSwitch under the
TCLink / TCIntf hierarchy. Future Open vSwitch QoS calls then create
new queues at an appropriate point in the hierarchy.