virtio: fix build for configs without dma-bufs

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: David Stevens <stevensd@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20200819031011.310180-1-stevensd@chromium.org


Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>