virtio-gpu: drop use_virgl_renderer

Now that we have separated the gl and non-gl code flows to two different
devices there is little reason turn on and off virglrenderer usage at
runtime.  The gl code can simply use virglrenderer unconditionally.

So drop use_virgl_renderer field and just do that.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 20210430113547.1816178-1-kraxel@redhat.com
Message-Id: <20210430113547.1816178-13-kraxel@redhat.com>