Support situation for nestedhvm

[Elliott Mitchell <ehem+xen@xxxxxxx>]

I ran into the nestedhvm via the following path.  I was considering the
feasibility of shedding tasks from a desktop onto a server running Xen.
I was looking at `man xl.cfg` and noticed "nestedhvm".

Since one of the tasks the computer handled was running other OSes in
fully simulated environments, this seemed to be something I was looking
for.  No where did I ever see anything hinting "This configuration option
is completely unsupported and risky to use".


For an option like this, additional steps should have been needed to
enable it.

First, on Xen's command-line there needs to be something along the
lines of "enable_unsupported=nestedhvm,others".

Second, in xl.cfg perhaps there should be an `enable_unsupported` option
which is a list of such options.

Third, perhaps a build-time configuration option too?


The issue is the above.  At no point did I realize I had crossed the
support boundary.  Things simply started exploding without any warnings.


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