spice: flip streaming video mode to off by default

Video streaming detection heuristics in spice-server have problems
keeping modern desktop animations (as done by gnome shell) and real
video playback apart.  This leads to jpeg compression artefacts on
your desktop, due to spice using mjpeg to send what it thinks is
a video stream.

Turn off video detection by default to avoid these artifacts.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Alon Levy <alevy@redhat.com>