spice.proto: add @outvar markup
spice.proto: add @outvar markup

With this we can reference pointer member with no naming conflicts

elmarco/spice-common,freedesktop-unofficial-mirror/spice__spice-common,fgouget/spice-common,fgouget/spice-common,elmarco/spice-common,freedesktop-unofficial-mirror/spice__spice-common,Fantu/spice-common,fgouget/spice-common,elmarco/spice-common,Fantu/spice-common,SPICE/spice-common,fgouget/spice-common,freedesktop-unofficial-mirror/spice__spice-common,freedesktop-unofficial-mirror/spice__spice-common,elmarco/spice-common,SPICE/spice-common,Fantu/spice-common,Fantu/spice-common,SPICE/spice-common