hw/block/nvme: split setup and register for namespace

hw/block/nvme: split setup and register for namespace

In NVMe, namespace is being attached to process I/O.  We register NVMe
namespace to a controller via nvme_register_namespace() during
nvme_ns_setup().  This is main reason of receiving NvmeCtrl object
instance to this function to map the namespace to a controller.

To make namespace instance more independent, it should be split into two
parts: setup and register.  This patch split them into two differnt
parts, and finally nvme_ns_setup() does not have nothing to do with
NvmeCtrl instance at all.

This patch is a former patch to introduce NVMe subsystem scheme to the
existing design especially for multi-path.  In that case, it should be
split into two to make namespace independent from a controller.

Signed-off-by: Minwoo Im <minwoo.im.dev@gmail.com>
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>