hw/block/nvme: move NvmeFeatureVal into hw/block/nvme.h

hw/block/nvme: move NvmeFeatureVal into hw/block/nvme.h

The NvmeFeatureVal does not belong with the spec-related data structures
in include/block/nvme.h that is shared between the block-level nvme
driver and the emulated nvme device.

Move it into the nvme device specific header file as it is the only
user of the structure. Also, remove the unused members.

Signed-off-by: Klaus Jensen <k.jensen@samsung.com>
Reviewed-by: Dmitry Fomichev <dmitry.fomichev@wdc.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20200706061303.246057-10-its@irrelevant.dk>