With the introduction of the nvme-subsystem device we are really
cluttering up the hw/block directory.

As suggested by Philippe previously, move the nvme emulation to hw/nvme.

Suggested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>