hw/block/nvme: fix shutdown/reset logic

hw/block/nvme: fix shutdown/reset logic

A shutdown is only about flushing stuff. It is the host that should
delete any queues, so do not perform a reset here.

Also, on shutdown, make sure that the PMR is flushed if in use.

Fixes: 368f4e752cf9 ("hw/block/nvme: Process controller reset and shutdown differently")
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Tested-by: Dmitry Fomichev <dmitry.fomichev@wdc.com>
Reviewed-by: Dmitry Fomichev <dmitry.fomichev@wdc.com>