virtual-source: Fix crash in combination with module-loopback

Similar to module-tunnel-sink-new, module-virtual-source did not create a rtpoll for the uplink sink. This lead to a crash when the uplink sink was used by module loopback, because module-loopback relies on the sink to provide a rtpoll. Additionally, the sink was not unlinked when the module was unloaded.

This patch fixes both issues. The rtpoll created is never run by the sink, so the patch is no real fix but just a workaround to make module-loopback happy.