channel-main: Add helper function for getting audio
channel-main: Add helper function for getting audio

Avoid repeating the same code and having a SpiceSession variable
defined when is not really needed.
Acked-by: Christophe Fergeau <05b95054a663c388a4e33d95828cb30f0370f8ef@redhat.com>

SPICE/spice-gtk,fgouget/spice-gtk,flexVDI/spice-gtk,fgouget/spice-gtk,SPICE/spice-gtk,SPICE/spice-gtk,flexVDI/spice-gtk,SPICE/spice-gtk,fgouget/spice-gtk