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/yuzu_cmd/yuzu.cpp

@@ -175,7 +175,7 @@ int main(int argc, char** argv) {
 
     Core::System& system{Core::System::GetInstance()};
     system.SetFilesystem(std::make_shared<FileSys::RealVfsFilesystem>());
-    Service::FileSystem::CreateFactories(system.GetFilesystem());
+    Service::FileSystem::CreateFactories(*system.GetFilesystem());
 
     SCOPE_EXIT({ system.Shutdown(); });