hw/block/nvme: Process controller reset and shutdown differently

hw/block/nvme: Process controller reset and shutdown differently

Controller reset ans subsystem shutdown are handled very much the same
in the current code, but some of the steps should be different in these
two cases.

Introduce two new functions, nvme_reset_ctrl() and nvme_shutdown_ctrl(),
to separate some portions of the code from nvme_clear_ctrl(). The steps
that are made different between reset and shutdown are that BAR.CC is not
reset to zero upon the shutdown and namespace data is flushed to
backing storage as a part of shutdown handling, but not upon reset.

Suggested-by: Klaus Jensen <k.jensen@samsung.com>
Signed-off-by: Dmitry Fomichev <dmitry.fomichev@wdc.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>