spice: support streaming-video parameter

spice: support streaming-video parameter

This adds a streaming-video=filter|all|off attribute. It is used to change
the behavior of video stream detection in spice, the default is filter (the
default for libvirt is not to specify it - the actual default is defined in
libspice-server.so).

Usage:

    <graphics type='spice' autoport='yes'>
      <streaming mode='off'/>
    </graphics>

Tested with the above and with tests/qemuxml2argvtest.

Signed-off-by: Alon Levy <alevy@redhat.com>