If you choose to enable the video streaming support of gstreamer,
please try to install addtional gst-plugins which matching the video codecs
One the client side:
mjpeg,x264 media-plugins/gst-plugins-libav:1.0
vpx media-plugins/gst-plugins-vpx:1.0
One the server side (which app-emulation/qemu running), you should also install
additional gstreamer plugins for the app-emulation/spice package:
mjpeg media-plugins/gst-plugins-libav:1.0
vpx media-plugins/gst-plugins-vpx:1.0
x264 media-plugins/gst-plugins-x264:1.0
