Add virtual chassis at device creation via GUI

[cpund]

There's already an issue for this, but it just hasn't had much conversation, and I've realized that technically that user should've posted a Discussion post first. Regardless, I'd almost view it as an oversight that the GUI doesn't allow for assignment of a virtual chassis at device creation, as the API already supports it. If that issue is approved, I also have a few lines of code that should permit selection of a virtual chassis at device creation already written. All you need is the following to match API parity:

• Virtual Chassis
• Vc position
• Vc priority

Position will be required when a virtual chassis is specified, but priority is optional. I wasn't sure how to implement the required field dynamically based on whether a virtual chassis is specified, but since the API already enforces that on the back end, it'll just spit out an error to the user saying they must select a position.

[cpund]

This was created for issue #9403 which has been closed with the addition of the merge from #9622 .