drm/virtio: fix byteorder handling in virtio_gpu_cmd_transfer_{from, to}_host_3d functions

Be consistent with the rest of the code base.
No functional change.

v2:
 - fix sparse warnings for virtio_gpu_cmd_transfer_to_host_2d call.
 - move convert_to_hw_box helper function.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20191023062539.11728-2-kraxel@redhat.com