bandwidth: Require network QoS if interface uses 'floor'

bandwidth: Require network QoS if interface uses 'floor'

By current implementation, network inbound is required in order
to use 'floor' for guaranteeing  minimal throughput. This is so,
because we want user to tell us the maximal throughput of the
network instead of finding out ourselves (and detect bogus values
in case of virtual interfaces). However, we are nowadays
requiring this only on documentation level. So if user starts a
domain with 'floor' set on one its interfaces, we silently ignore
the setting. We should error out instead.