Don't read from disk when checking volume type

Should be faster than relying on the OS to cache the magic words.
Also gets rid of the odd recursive call in VolumeDirectory.

Source/Core/DiscIO/VolumeWad.h

@@ -32,6 +32,7 @@ public:
 	std::vector<std::string> GetNames() const override;
 	u32 GetFSTSize() const override               { return 0; }
 	std::string GetApploaderDate() const override { return "0"; }
+	bool IsWadFile() const override;
 	ECountry GetCountry() const override;
 	u64 GetSize() const override;
 	u64 GetRawSize() const override;