As far as I can read the Apple License (and I am not a lawyer), installing Apple operating systems in a virtualized environment is permitted by the terms of the license as long as the virtualized environment is running on Apple-branded hardware. Running a virtualized system on non-Apple hardware is "explicitly unlicensed use of product".
If we want to keep that as our bright line test (and I'm indifferent to that as a standard, but it is our current one), then I'd say questions about installing OS X in a virtualized environment in general are on-topic here, because they could apply to a licensed use of OS X. If one can run Linux on a Mac (and a quick Google search indicates that this seems to be possible), then running OS X virtualized on a Mac that is running Linux would be a licensed use of the product, and thus on-topic for the site.
Questions specific to running OS X virtualized on Linux on, say, a Dell or a Wang or ENIAC would be off-topic because they explicitly violate the license agreement.
So my reading of the current standards is that because one could run Linux on a Mac and then legitimately virtualize OS X on top of that, the question is on-topic and needs no update to the FAQ.
Going through your examples,
the first one is on-topic as is; Ubuntu can run on a Mac, so an end user entirely within the scope of Apple's license could be helped by this answer.
The second one is explicitly run on a Mac. The Apple terms say the software is licensed to run on Apple hardware, and that's what's being done.
The third one is off-topic; it's asking specifically about getting it to run on a HP computer. It might be editable to be on-topic, but if the problem is specific to the drivers needed for the HP hardware, it's off-topic.
The fourth one is on-topic. Windows can run on a Mac, so this answer could apply to someone running Mac OS X on Apple hardware.
The fifth one is leaning toward off-topic as written, but edit "I have a Windows PC" to "I have a computer running Windows" and we're back in on-topic territory.
The sixth one is clearly on-topic; it's asking what the license allows, rather than asking how to do something not permitted.