Validate `guest_mac` when supplied

**Describe the solution you'd like:**
If the guest mac is supplied in the create microvm api request we should validate the following:

- Its the correct format
- Multicast bit isn't set
- Its not the same as any other mac address in the request
- For a macvtap interface check that there isn't an existing interface with the name mac address on the host already

Q: do we care about the global/locally unique bit

**Why do you want this feature:**

To provide better feedback to the user

**Anything else you would like to add:**
[Miscellaneous information that will assist in solving the issue.]

Start by locating the create microVM API request handling and where guest_mac values are processed; the issue names no files, tests, or entry points. Check how macvtap interfaces are identified on the host and define completion around format, multicast, duplicate, and existing-interface validation, including the unresolved global/local uniqueness question.

Written by the indexing model from the issue text.