[XMA] Fix dedicated thread stalls by blocking guest audio thread

Includes various other thread safety and timing related adjustments

src/xenia/apu/xma_context.cc

@@ -35,7 +35,9 @@ extern "C" {
 namespace xe {
 namespace apu {
 
-XmaContext::XmaContext() = default;
+XmaContext::XmaContext()
+    : work_completion_event_(
+          xe::threading::Event::CreateAutoResetEvent(false)) {}
 
 XmaContext::~XmaContext() {}