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

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: https://gitlab.freedesktop.org/pulseaudio/pulseaudio/issues/775