Fix bus index not being serialized in GetBusInfo

This is embarrassing. Because the bus index was not being serialized,
all lookups were done for the bus with index 0. This meant that
sidechaining in Renoise didn't work because Renoise only allows
sidechaining to `kAux` busses and the first bus is always marked as
`kMain`. This would also cause Ardour to crash or freeze more often then
it should because while it does not support arbitrary bus I/O
configurations, it does support plugins with both a `kMain` and a `kAux`
input bus but since we would never get `kAux` busses Ardour just didn't
pass any buffers for the sidechain input.

CHANGELOG.md

@@ -6,6 +6,14 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic
 Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Fixed
+
+- Fix bus information queries being performed for the wrong bus index. This
+  fixes VST3 sidechaining in _Renoise_, and prevents a number of VST3 plugins
+  with a sidechain input from causing _Ardour_ and _Mixbus_ to freeze.
+
 ## [3.0.1] - 2020-02-26
 
 ### Changed

src/common/serialization/vst3/plugin/component.h

@@ -175,6 +175,7 @@ class YaComponent : public Steinberg::Vst::IComponent {
             s.value8b(instance_id);
             s.value4b(type);
             s.value4b(dir);
+            s.value4b(index);
         }
     };