bridge_driver: Introduce networkBandwidthChangeAllowed
    
    When a domain vNIC's bandwidth is to be changed (at runtime) it is
    possible that guaranteed minimal bandwidth (@floor) will change too.
    Well, so far it is, because we still don't have an implementation that
    allows setting it dynamically, so it's effectively erased on:
    
        #virsh domiftune $dom vnet0 --inbound 0
    
    However, that's slightly unfortunate. We do some checks on domain
    startup to see if @floor can be guaranteed. We ought do the same if
    QoS is changed at runtime.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>