hw/block/nvme: fix legacy namespace registration

hw/block/nvme: fix legacy namespace registration

Moving namespace registration to the nvme-ns realization function had
the unintended side-effect of breaking legacy namespace registration.
Fix this.

Fixes: 15d024d4aa9b ("hw/block/nvme: split setup and register for namespace")
Reported-by: Alexander Graf <agraf@csgraf.de>
Cc: Minwoo Im <minwoo.im.dev@gmail.com>
Tested-by: Alexander Graf <agraf@csgraf.de>
Reviewed-by: Minwoo Im <minwoo.im.dev@gmail.com>
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>