nvme multipath support V2

[Christoph Hellwig]

On Thu, Sep 21, 2017 at 07:23:45AM +0200, Johannes Thumshirn wrote:
> Ah OK, we maybe should update nvme-cli to recognize it as well then. I just
> looked at the output of nvme list and obviously didn't find it.

Overloading the new per-subsystem nodes into nvme list would be
very confusing I think.  We could add a new command to list subsystems
instead of controllers, if just doing a ls in /dev or sysfs is too hard.