pulse: default volume should be the last from the stream

Submitted by Victor Toso @victortoso

Link to original bug (#766020)

Description

When I use pulseaudio backend in Spice, I retrieve the last volume from the stream using pa_ext_stream_restore_read[0] and I can sync volume between machines before any audio starts.

[0] http://b045.isibrno.cz/share/doc/pulseaudio-libs-devel-0.9.21/html/ext-stream-restore_8h.html#a3e83c1723e241f9e308be4097e1d3973

With pulsesink/pulsesrc, if I try to get the volume before the stream has started, I'll get the default "1.0" [1].

[1] https://cgit.freedesktop.org/gstreamer/gst-plugins-good/tree/ext/pulse/pulsesink.c#n2612

I think we could use pa_ext_stream_restore if we have the stream-name and return that instead of 100%