def get_config(self, instance, vif, image_meta, inst_type):

        conf = vconfig.LibvirtConfigGuestInterface()

        # Default to letting libvirt / the hypervisor choose the model

        model = None

        driver = None

        # If the user has specified a 'vif_model' against the

        # image then honour that model

        if image_meta:

            vif_model = image_meta.get('properties',

                                       {}).get('hw_vif_model')

            if vif_model is not None:

                model = vif_model

        # Else if the virt type is KVM/QEMU, use virtio according

        # to the global config parameter

        if (model is None and

            CONF.libvirt.virt_type in ('kvm', 'qemu') and

                    CONF.libvirt.use_virtio_for_bridges):

            model = "virtio"

        # Workaround libvirt bug, where it mistakenly

        # enables vhost mode, even for non-KVM guests

        if model == "virtio" and CONF.libvirt.virt_type == "qemu":

            driver = "qemu"

if model == "virtio" and CONF.libvirt.virt_type == "qemu":

            driver = "qemu"

driver = "qemu"//增加此句，代表永远为qemu