filesystem: Make CreateFactories() and InstallInterface() take a VfsFilesystem instance by reference

Neither of these functions alter the ownership of the provided pointer,
so we can simply make the parameters a reference rather than a direct
shared pointer alias. This way we also disallow passing incorrect memory values like
nullptr.

src/core/core.cpp

@@ -148,7 +148,7 @@ struct System::Impl {
         telemetry_session = std::make_unique<Core::TelemetrySession>();
         service_manager = std::make_shared<Service::SM::ServiceManager>();
 
-        Service::Init(service_manager, virtual_filesystem);
+        Service::Init(service_manager, *virtual_filesystem);
         GDBStub::Init();
 
         renderer = VideoCore::CreateRenderer(emu_window);