Add 'devname' to nova.network.model.VIF class

Add a 'devname' field to nova.network.model.VIF to allow network
drivers to specify a desired TAP device name. Update the Quantum
driver to set a device name, and update the libvirt VIF drivers
to honour it.

Blueprint: libvirt-vif-driver
Change-Id: Ia8dd546af36e7f6c22efd7c696fb58b37aac0061
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Merge "Add 'devname' to nova.network.model.VIF class"