virtio: Support prime objects vmap/vunmap

virtio: Support prime objects vmap/vunmap

Implement vmap/vunmap so we can export dmabufs to
other drivers, such as video4linux.

Tested with a virtio-gpu / vivid (virtual capture driver)
pipeline, where the vivid driver imports the dmabufs exported
by virtio-gpu.

Note that dma_buf_vmap() does its own vmap counting, so
it's not needed to take care of it in the driver.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20180925161606.17980-4-ezequiel@collabora.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>