Fixes #6134 - Discovered hosts prefix can be configured

That's right. We need to check this, it's the purpose of the prefix actually. Please check:

* it does start with A-Za-z
* it is not empty

If any fails, please fallback to `mac`. If it's possible, cover with unit tests for the edge cases. The code looks good, once we have the check, I am going to test. Ty.