docs: def interface_by_macaddr(self, macaddr):
          '''
          Given a MAC address, return the interface that 'owns' this address
          '''
          macaddr = EthAddr(macaddr)
          for devname,iface in self._devinfo.items():
              if iface.ethaddr == macaddr:
                  return iface
          raise KeyError("No device has MAC address {}".format(macaddr))