nvme: use menu Kconfig interface

Add a menu interface for NVME host and target support so that it is
presented to users more like other Kconfig symbols.
This makes the Device Driver menu less cluttered (easier to read)
and keeps all of these symbols grouped together.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>