Description of problem:
In 4.13 we can download manifests from VMExport, virtctl makes the process much easier

Version-Release number of selected component (if applicable):
4.13

How reproducible:
Always

Steps to Reproduce:
1. Create a VMExport, try to download a manifest:
   virtctl vmexport download export --manifest --vm=example

Actual results:
Command not supported

Expected results:
Command supported, manifest downloaded