Update virtual_hardware.md

[Ben-Chapman]

This is a minor update to the Video and Graphics Device section, better clarifying not attaching a graphics device to a VirtualMachineInstance

What this PR does / why we need it:

• This PR is a minor documentation update, adding clarity to a configuration description

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged): Fixes #

Special notes for your reviewer:

Checklist

This checklist is not enforcing, but it's a reminder of items that could be relevant to every PR. Approvers are expected to review this list.

• [ ] Design: A design document was considered and is present (link) or not required
• [ ] PR: The PR description is expressive enough and will help future contributors
• [ ] Code: Write code that humans can understand and Keep it simple
• [ ] Refactor: You have left the code cleaner than you found it (Boy Scout Rule)
• [ ] Upgrade: Impact of this change on upgrade flows was considered and addressed if required
• [ ] Testing: New code requires new unit tests. New features and bug fixes require at least on e2e test
• [x] Documentation: A user-guide update was considered and is present (link) or not required. You want a user-guide update if it's a user facing feature / API change.
• [ ] Community: Announcement to kubevirt-dev was considered

Release note:

NONE