bridge_driver: Introduce networkBandwidthChangeAllowed

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>