[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.cc

@@ -0,0 +1,62 @@
+/**
+ ******************************************************************************
+ * 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. *
+ ******************************************************************************
+ */
+
+#include "xenia/vfs/devices/null_device.h"
+
+#include "xenia/base/filesystem.h"
+#include "xenia/base/logging.h"
+#include "xenia/base/math.h"
+#include "xenia/kernel/xfile.h"
+#include "xenia/vfs/devices/null_entry.h"
+
+namespace xe {
+namespace vfs {
+
+NullDevice::NullDevice(const std::string& mount_path,
+                       const std::initializer_list<std::string>& null_paths)
+    : Device(mount_path), null_paths_(null_paths), name_("NullDevice") {}
+
+NullDevice::~NullDevice() = default;
+
+bool NullDevice::Initialize() {
+  auto root_entry = new NullEntry(this, nullptr, mount_path_);
+  root_entry->attributes_ = kFileAttributeDirectory;
+  root_entry_ = std::unique_ptr<Entry>(root_entry);
+
+  for (auto path : null_paths_) {
+    auto child = NullEntry::Create(this, root_entry, path);
+    root_entry->children_.push_back(std::unique_ptr<Entry>(child));
+  }
+  return true;
+}
+
+void NullDevice::Dump(StringBuffer* string_buffer) {
+  auto global_lock = global_critical_region_.Acquire();
+  root_entry_->Dump(string_buffer, 0);
+}
+
+Entry* NullDevice::ResolvePath(const std::string_view path) {
+  XELOGFS("NullDevice::ResolvePath({})", path);
+
+  auto root = root_entry_.get();
+  if (path.empty()) {
+    return root_entry_.get();
+  }
+
+  for (auto& child : root->children()) {
+    if (!strcasecmp(child->path().c_str(), path.data())) {
+      return child.get();
+    }
+  }
+
+  return nullptr;
+}
+
+}  // namespace vfs
+}  // namespace xe