Fix vcd disk list command

[skydion]

Usage: vcd disk list [OPTIONS]
Try "vcd disk list -h" for help.

Error: int() argument must be a string, a bytes-like object or a number, not 'NoneType'

To help us process your pull request efficiently, please include:

• (Required) Short description of changes in the PR topic line

• (Required) Detailed description of changes include tests and
  documentation. If the pull request contains multiple commits with
  detailed messages, refer to those instead

• (Optional) Names of reviewers using @ sign + name

[vmwclabot]

@skydion, you must sign our contributor license agreement before your changes are merged. Click here to sign the agreement. If you are a VMware employee, read this for further instruction.

[vmwclabot]

@skydion, your company's legal contact has approved your signed contributor license agreement. It will also be reviewed by VMware, but the merge can proceed.

[vmwclabot]

@skydion, VMware has approved your signed contributor license agreement.

[MarioAlexis]

This issue is a real pain. Especially for automation CI/CD. Any plan to merge it soon ?