network: use bandwidth from portgroup when appropriate

network: use bandwidth from portgroup when appropriate

The bandwidth plug and unplug functions were assuming that an
interface's bandwidth setting was always specified directly in the
domain's <interface> definition, but that's not necessarily true - it
could have been obtained from a <portgroup> definition in the network
definition. This patch fixes those functions to use
virDomainNetGetActualBandwidth(), which gets the bandwidth pointer
from iface->data.network.actual if it exists, otherwise returns
iface->bandwidth.