[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_device.h

@@ -0,0 +1,57 @@
+/**
+ ******************************************************************************
+ * 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_DEVICE_H_
+#define XENIA_VFS_DEVICES_NULL_DEVICE_H_
+
+#include <string>
+
+#include "xenia/vfs/device.h"
+
+namespace xe {
+namespace vfs {
+
+class NullEntry;
+
+class NullDevice : public Device {
+ public:
+  NullDevice(const std::string& mount_path,
+             const std::initializer_list<std::string>& null_paths);
+  ~NullDevice() override;
+
+  bool Initialize() override;
+  void Dump(StringBuffer* string_buffer) override;
+  Entry* ResolvePath(const std::string_view path) override;
+
+  bool is_read_only() const override { return false; }
+
+  const std::string& name() const override { return name_; }
+  uint32_t attributes() const override { return 0; }
+  uint32_t component_name_max_length() const override { return 40; }
+
+  uint32_t total_allocation_units() const override { return 0x10; }
+  uint32_t available_allocation_units() const override { return 0x10; }
+
+  // STFC/cache code seems to require the product of the next two to equal
+  // 0x10000
+  uint32_t sectors_per_allocation_unit() const override { return 0x80; }
+
+  // STFC requires <= 0x1000
+  uint32_t bytes_per_sector() const override { return 0x200; }
+
+ private:
+  std::string name_;
+  std::unique_ptr<Entry> root_entry_;
+  std::vector<std::string> null_paths_;
+};
+
+}  // namespace vfs
+}  // namespace xe
+
+#endif  // XENIA_VFS_DEVICES_NULL_DEVICE_H_