Add support for nvme disk detection

Add support for nvme disk detection

This treats /dev/nvme.. devices the same way as /dev/sd... devices.  The
motivation behind this is that whole disk detection did not work on nvme
SSDs without that, because it DKC_UNKNOWN was returned for such devices.

Perhaps there should be a separate DKC_ type for this, but I don't know
enough about the code to know the implications of that.

Reviewed-by: Don Brady <don.brady@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: timor <timor.dd@googlemail.com>
Closes #7304