Add support for nvme based devids

Add support for nvme based devids

Adds a devid for nvme devices. This is very similar to how the
other 'bus' (scsi|sata|usb) devids are generated. The devid
resides in a name/value pair in the leaf vdevs in a zpool config.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Don Brady <don.brady@delphix.com>
Closes #7356