GraphicHw.gl_flushed was introduced to notify the
device (vhost-user-gpu) that the GL resources (the display scanout) are
no longer needed.

It was decoupled from QEMU own gl-blocking mechanism, but that
difference isn't helping. Instead, we can reuse QEMU gl-blocking and
notify virtio_gpu_gl_flushed() when unblocking (to unlock
vhost-user-gpu).

An extra block/unblock is added arount dpy_gl_update() so existing
backends that don't block will have the flush event handled. It will
also help when there are no backends associated.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>