hw/nvme: fix nvme hotplugging

hw/nvme: fix nvme hotplugging

Commit cd59f50ab017 caused a regression on nvme hotplugging for devices
with an implicit nvm subsystem.

The nvme-subsys device was incorrectly left with being marked as
non-hotpluggable. Fix this.

Cc: qemu-stable@nongnu.org
Reported-by: Stéphane Graber <stgraber@stgraber.org>
Tested-by: Stéphane Graber <stgraber@stgraber.org>
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2950
Fixes: cd59f50ab017 ("hw/nvme: always initialize a subsystem")
Reviewed-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>