Io: Provide directory existence with listing.

Sometimes, you need to tell the difference between an empty directory and
one that doesn't exist at all.  We can do this in a single call.

Core/FileSystems/VirtualDiscFileSystem.h

@@ -40,7 +40,7 @@ public:
 	bool     OwnsHandle(u32 handle) override;
 	int      Ioctl(u32 handle, u32 cmd, u32 indataPtr, u32 inlen, u32 outdataPtr, u32 outlen, int &usec) override;
 	PSPDevType DevType(u32 handle) override;
-	std::vector<PSPFileInfo> GetDirListing(std::string path) override;
+	std::vector<PSPFileInfo> GetDirListing(const std::string &path, bool *exists = nullptr) override;
 	FileSystemFlags Flags() override { return FileSystemFlags::UMD; }
 	u64  FreeSpace(const std::string &path) override { return 0; }