Return the number of network interfaces with id ``connection``.

def getNumNetworkWithConnection(self, connection):
        """Return the number of network interfaces with id ``connection``."""

        i = 0
        while True:
            value = self.getValue("net_interface.%d.connection" % i, None)
            if not value:
                return None
            if value == connection:
                return i
            i += 1