[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_file.cc

@@ -0,0 +1,52 @@
+/**
+ ******************************************************************************
+ * 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_file.h"
+
+#include "xenia/vfs/devices/null_entry.h"
+
+namespace xe {
+namespace vfs {
+
+NullFile::NullFile(uint32_t file_access, NullEntry* entry)
+    : File(file_access, entry) {}
+
+NullFile::~NullFile() = default;
+
+void NullFile::Destroy() { delete this; }
+
+X_STATUS NullFile::ReadSync(void* buffer, size_t buffer_length,
+                            size_t byte_offset, size_t* out_bytes_read) {
+  if (!(file_access_ & FileAccess::kFileReadData)) {
+    return X_STATUS_ACCESS_DENIED;
+  }
+
+  return X_STATUS_SUCCESS;
+}
+
+X_STATUS NullFile::WriteSync(const void* buffer, size_t buffer_length,
+                             size_t byte_offset, size_t* out_bytes_written) {
+  if (!(file_access_ &
+        (FileAccess::kFileWriteData | FileAccess::kFileAppendData))) {
+    return X_STATUS_ACCESS_DENIED;
+  }
+
+  return X_STATUS_SUCCESS;
+}
+
+X_STATUS NullFile::SetLength(size_t length) {
+  if (!(file_access_ & FileAccess::kFileWriteData)) {
+    return X_STATUS_ACCESS_DENIED;
+  }
+
+  return X_STATUS_SUCCESS;
+}
+
+}  // namespace vfs
+}  // namespace xe