nvme-core: add method to check for an I/O controller

Add nvme_is_io_ctrl() to check if the controller is of type I/O
controller. Uses negative logic by excluding an administrative
controller and a discovery controller.

Signed-off-by: Kamaljit Singh <kamaljit.singh@opensource.wdc.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>