hw/nvme: reattach subsystem namespaces on hotplug

hw/nvme: reattach subsystem namespaces on hotplug

With commit 5ffbaeed16 ("hw/nvme: fix controller hot unplugging")
namespaces get moved from the controller to the subsystem if one
is specified.
That keeps the namespaces alive after a controller hot-unplug, but
after a controller hotplug we have to reconnect the namespaces
from the subsystem to the controller.

Fixes: 5ffbaeed16 ("hw/nvme: fix controller hot unplugging")
Cc: Klaus Jensen <k.jensen@samsung.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Hannes Reinecke <hare@suse.de>
[k.jensen: only attach to shared and non-detached namespaces]
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>