libvirt_network_bandwidth: fix bug for AttributeError

since the class Interface has been updated, the setting about network
bandwidth for interface will suffer from AttributeError.as following,
07:11:09 ERROR| AttributeError: Key 'bandwidth_inbound' not found in super class attributes or in ('source', 'mac_address', 'bandwidth', 'model', 'link_state', 'driver', 'address', 'type_name', 'device_tag', 'xml', 'virsh', 'xmltreefile', 'validates', 'get_bandwidth_inbound', 'set_bandwidth_inbound', 'del_bandwidth_inbound')

This patch fixes the problem.