Add support for virtual source

A virtual source is usually implemented with a null-sink, it looks
like a source to pulseaudio clients but just forwards what it gets
as input.

Make sure the port names make sense.

You can use the null sink now as:

pactl load-module module-null-sink sink_name=source object.linger=1
     media.class=Audio/Source/Virtual channel_map=FL,FR,RL,RR

     This creates a node with "input" and "capture" ports and looks
     like a virtual source for pulseaudio clients.

pactl load-module module-null-sink sink_name=source object.linger=1
     media.class=Audio/Sink channel_map=FL,FR,RL,RR

     This creates a node with "playback" and "monitor" ports and looks
     like a virtual sink for pulseaudio clients.

pactl load-module module-null-sink sink_name=source object.linger=1
     media.class=Audio/Duplex channel_map=FL,FR,RL,RR

     This creates a node with "playback" and "capture" ports and looks
     like a virtual source and sink for pulseaudio clients.