virtio-scsi: Move DEFINE_VIRTIO_SCSI_FEATURES to virtio-scsi

So far virtio-scsi-device can't expose host features to guest while
using virtio-mmio because it doesn't set DEFINE_VIRTIO_SCSI_FEATURES on
backend or transport.

The host features belong to the backends while virtio-scsi-pci,
virtio-scsi-s390 and virtio-scsi-ccw set the DEFINE_VIRTIO_SCSI_FEATURES
on transports. But they already have the ability to forward property
accesses to the backend child. So if we move the host features to
backends, it doesn't break the backwards compatibility for them and
make host features work while using virtio-mmio.

Move DEFINE_VIRTIO_SCSI_FEATURES to the backend virtio-scsi. The
transports just sync the host features from backends.

Signed-off-by: Shannon Zhao <zhaoshenglong@huawei.com>
Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
