enable generic interface (/dev/ngX) for unknown command sets v3

[Sagi Grimberg]

> Hi all,
> 
> this is a version of the patch from Joel rebased on a bunch of cleanups
> from me that I think make the layering a lot cleaner.  Let me know what
> everyone things.
> 
> Changes since v2:
>   - add a patch to fix a previous regression in handling of unsupported
>     ZNS configs

Woops, just noticed this one...

Still,
Reviewed-by: Sagi Grimberg <sagi at grimberg.me>