hw/block/nvme: fix allocated namespace list to 256

hw/block/nvme: fix allocated namespace list to 256

Expand allocated namespace list (subsys->namespaces) to have 256 entries
which is a value lager than at least NVME_MAX_NAMESPACES which is for
attached namespace list in a controller.

Allocated namespace list should at least larger than attached namespace
list.

	n->num_namespaces = NVME_MAX_NAMESPACES;

The above line will set the NN field by id->nn so that the subsystem
should also prepare at least this number of namespace list entries.

Signed-off-by: Minwoo Im <minwoo.im.dev@gmail.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Klaus Jensen <k.jensen@samsung.com>
Tested-by: Klaus Jensen <k.jensen@samsung.com>
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>