enable generic interface (/dev/ngX) for unknown command sets v2

[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 v1:
>   - log consistent messages when only passthrough is supported for
>     namespaces

Looks good Christoph,

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