Fix LibvirtGenericVIFDriver.get_config() for quota

When Nova is used with Neutron and Open vSwitch, network quotas on the
bandwidth are ignored by libvirt: the "<bandwidth>" tag is not generated in
libvirt.xml of the virtual machine. This change fixes this issue but also for
the network types: "bridge", "network" and "direct".

Commands to create the flavor with download limited to 1 MB/sec and create a
virtual machine with this flavor:
---
nova flavor-create --ephemeral=1 victor_test_vif 50 256 1 1
nova flavor-key victor_test_vif set quota:vif_inbound_average=1000
nova boot --flavor=victor_test_vif --image=cirros-0.3.1-x86_64-uec victor_test
---

Commands to upload a file to this virtual machine with SCP to check that the
bandwidth is limited to 1 MB/sec (replace 10.0.0.3 with the IP of the virtual
machine):
---
ip netns
sudo ip netns exec qrouter-128db593-a0db-40c3-84c7-e6383d40c75f bash

dd if=/dev/urandom of=random10MB bs=1024 count=10240
scp random10M cirros@10.0.3:
---

Change-Id: I0f0111fc79fe90900e38df022034b208f1129088
Closes-Bug: #1254664
Merge "Fix LibvirtGenericVIFDriver.get_config() for quota"