drm/virtio: add create_handle support.

Add create_handle support to virtio fb. Without this, screenshot tool
in chromium OS can't work.

Signed-off-by: Lepton Wu <ytht.net@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20171108184209.46211-1-ytht.net@gmail.com


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