network: remove the virDomainNetBandwidthChangeAllowed callback

network: remove the virDomainNetBandwidthChangeAllowed callback

The current qemu driver code for changing bandwidth on a NIC first asks
the network driver if the change is supported, then changes the
bandwidth on the VIF, and then tells the network driver to update the
bandwidth on the bridge.

This is potentially racing if a parallel API call causes the network
driver to allocate bandwidth on the bridge between the check and the
update phases.

Change the code to just try to apply the network bridge update
immediately and rollback at the end if something failed.

Reviewed-by: Laine Stump <laine@laine.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>