bandwidth: add new 'floor' attribute

bandwidth: add new 'floor' attribute

This is however supported only on domain interfaces with
type='network'. Moreover, target network needs to have at least
inbound QoS set. This is required by hierarchical traffic shaping.

From now on, the required attribute for <inbound/> is either 'average'
(old) or 'floor' (new). This new attribute can be used just for
interfaces type of network (<interface type='network'/>) currently.