Use stereo when available for sample rate probing

Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>
Use stereo in audiostreamer when audio devices support it

Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>
Add crude stereo support to audiostreamer_pa

We always run mono internally but it is useful to open devices in
stereo.  This way stereo devices have audio on both channels, which is
what users expect.

Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>