Description of problem:
create a vm from catalog and try to use "Using an existing PVC" as VM disk, it failed with error:
Error "spec.template.spec.volumes[0] must have exactly one source type set" for field "spec.template.spec.volumes[0]".

Version-Release number of selected component (if applicable):
v4.12.0

How reproducible:

Steps to Reproduce:
1. prepare a pvc
2. create a vm from catalog and try to use "Using an existing PVC" as VM disk
3.

Actual results:

Expected results:

Additional info: