[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.cc

@@ -0,0 +1,55 @@
+/**
+ ******************************************************************************
+ * 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_entry.h"
+
+#include "xenia/base/filesystem.h"
+#include "xenia/base/logging.h"
+#include "xenia/base/mapped_memory.h"
+#include "xenia/base/math.h"
+#include "xenia/base/string.h"
+#include "xenia/vfs/device.h"
+#include "xenia/vfs/devices/null_file.h"
+
+namespace xe {
+namespace vfs {
+
+NullEntry::NullEntry(Device* device, Entry* parent, std::string path)
+    : Entry(device, parent, path) {}
+
+NullEntry::~NullEntry() = default;
+
+NullEntry* NullEntry::Create(Device* device, Entry* parent,
+                             const std::string& path) {
+  auto entry = new NullEntry(device, parent, path);
+
+  entry->create_timestamp_ = 0;
+  entry->access_timestamp_ = 0;
+  entry->write_timestamp_ = 0;
+
+  entry->attributes_ = kFileAttributeNormal;
+
+  entry->size_ = 0;
+  entry->allocation_size_ = 0;
+  return entry;
+}
+
+X_STATUS NullEntry::Open(uint32_t desired_access, File** out_file) {
+  if (is_read_only() && (desired_access & (FileAccess::kFileWriteData |
+                                           FileAccess::kFileAppendData))) {
+    XELOGE("Attempting to open file for write access on read-only device");
+    return X_STATUS_ACCESS_DENIED;
+  }
+
+  *out_file = new NullFile(desired_access, this);
+  return X_STATUS_SUCCESS;
+}
+
+}  // namespace vfs
+}  // namespace xe