[VFS] Add NullDevice (returns success for all calls), handle \Device\Harddisk0\ with it

XMountUtilityDrive code tries reading/writing from \Device\Harddisk0\Cache0 / Cache1 / Partition0, NullDevice handling \Device\Harddisk0 will make that code think that the reads/writes were successful, so the utility-drive mount can proceed without failing.

src/xenia/vfs/devices/null_entry.h

@@ -0,0 +1,42 @@
+/**
+ ******************************************************************************
+ * Xenia : Xbox 360 Emulator Research Project                                 *
+ ******************************************************************************
+ * Copyright 2021 Ben Vanik. All rights reserved.                             *
+ * Released under the BSD license - see LICENSE in the root for more details. *
+ ******************************************************************************
+ */
+
+#ifndef XENIA_VFS_DEVICES_NULL_ENTRY_H_
+#define XENIA_VFS_DEVICES_NULL_ENTRY_H_
+
+#include <string>
+
+#include "xenia/base/filesystem.h"
+#include "xenia/vfs/entry.h"
+
+namespace xe {
+namespace vfs {
+
+class NullDevice;
+
+class NullEntry : public Entry {
+ public:
+  NullEntry(Device* device, Entry* parent, std::string path);
+  ~NullEntry() override;
+
+  static NullEntry* Create(Device* device, Entry* parent,
+                           const std::string& path);
+
+  X_STATUS Open(uint32_t desired_access, File** out_file) override;
+
+  bool can_map() const override { return false; }
+
+ private:
+  friend class NullDevice;
+};
+
+}  // namespace vfs
+}  // namespace xe
+
+#endif  // XENIA_VFS_DEVICES_NULL_ENTRY_H_