'Update instance vnic index.
When the \'VNIC index\' extension is supported this method will update
the vnic index of the instance on the port. A instance may have more
than one vnic.
:param context: The request context.
:param instance: nova.objects.instance.Instance object.
:param vif: The VIF in question.
:param index: The index on the instance for the VIF.'

def update_instance_vnic_index(self, context, instance, vif, index):

    pass
