Add Spice graphics gl attribute. qemu 2.6 should have -spice gl=on argument to
enable opengl rendering context (patches on the ML). This is necessary to
actually enable virgl rendering.

Add a qemuxml2argv test for virtio-gpu + spice with virgl.
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>