KVM - config_drive <Image UUID> doesn't create disk.config

I'm testing config_drive function using latest trunk code.

Launch one server with config_drive set to <image-uuid>.

$nova boot --flavor 1 --image a1f827d7-9726-4343-b52a-b8a2bf4cb594 --key_name openstack --config-drive 80495421-ed25-4fc1-918f-88d6ebf5f3dd iso-config-drive

If the value is set to “image-UUID”, it builds a volume from the image and attaches it.

It should create disk.config file of size equal to image size on the compute node inside the instance folder. But this file doesn't exists in the instance folder.
root@ubuntu-precise-002:/opt/stack/nova/instances/instance-00000017# ls -lrt
total 13708
-rw-rw-r-- 1 rohit libvirtd 1363 Jul 24 14:40 libvirt.xml
-rw-rw-r-- 1 libvirt-qemu kvm 4731440 Jul 24 14:40 kernel
-rw-rw-r-- 1 libvirt-qemu kvm 2254249 Jul 24 14:40 ramdisk
-rw-rw---- 1 libvirt-qemu kvm 16007 Jul 24 14:41 console.log
-rw-r--r-- 1 libvirt-qemu kvm 7143424 Jul 24 14:41 disk

I only see one disk attached to the instance in the libvirt.xml

<disk type="file" device="disk">
      <driver type="qcow2" cache="none"/>
      <source file="/opt/stack/nova/instances/instance-00000017/disk"/>
      <target bus="virtio" dev="vda"/>
</disk>

After putting debug trace in nova.virt.libvirt.driver.py, I could see config_drive_id is None. There is no config_drive_id column in the instance db table and also this config_drive_id is not sent to the compute manager in the RPC call from scheduler (filter_scheduler).

Is this a bug?

Note: If I set config_drive to True, then I could see disk.config inside instance folder of size 64MB as expected.