Horizon should autofill Source tab for instance when launching it from volume

Reproduced on recent devstack master.

Steps to reproduce:

1. Create a bootable volume from cirros image. 1 Gb is enough
2. Select created volume and choose "Launch as Instance"
3. Define instance name, allocate flavor and network in corresponding tabs
4. Launch instance

Horizon should autofill Source tab for instance correctly when launching it from volume.
By default “image” option is selected, leading to error: Block Device Mapping is Invalid: Missing device UUID. (HTTP 400) (Request-ID: req-****some-uuid****)

Expected Result:
Instance created successfully

Actual Result:
Instance creation fails with following error: Block Device Mapping is Invalid: Missing device UUID. (HTTP 400) (Request-ID: req-****some-uuid****)