vfs: Make type hierarchy objects classes instead of structs

struct should be used when the data type is very simple or otherwise has
no invariants associated with it. Given these are used to form a
hierarchy, class should be used instead.

src/core/file_sys/vfs_offset.h

@@ -15,7 +15,8 @@ namespace FileSys {
 // Similar to seeking to an offset.
 // If the file is writable, operations that would write past the end of the offset file will expand
 // the size of this wrapper.
-struct OffsetVfsFile : public VfsFile {
+class OffsetVfsFile : public VfsFile {
+public:
     OffsetVfsFile(std::shared_ptr<VfsFile> file, size_t size, size_t offset = 0,
                   std::string new_name = "", VirtualDir new_parent = nullptr);