Add egl-headless user interface.  It doesn't provide a real user
interface, it only provides opengl support using drm render nodes.
It will copy back the bits rendered by the guest using virgl back
to a DisplaySurface and kick the usual display update code paths,
so spice and vnc and screendump can pick it up.

Use it this way:
  qemu -display egl-headless -vnc $display
  qemu -display egl-headless -spice gl=off,$args

Note that you should prefer native spice opengl support (-spice
gl=on) if possible because that delivers better performance.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Message-id: 20170505104101.30589-7-kraxel@redhat.com