virtual-source: Don't process the rewind during requesting it

The request_rewind() callback of the uplink sink called pa_sink_process_rewind(), which is not allowed. Things are supposed to happen so that first a rewind is requested, and then during the next rtpoll loop the sink will process the request. Calling pa_sink_process_rewind() during the request phase caused a crash.

Having a request_rewind() callback is completely unnecessary, because it's only useful for forwarding the request to a downstream sink. In this case there is no downstream sink.

I also set max_rewind to 0, because the sink doesn't support rewinding.

Fixes: #775 (closed)