Reads one joystick device file inside the VM, for Gamepad.

start_link/2 starts a process, linked to the caller, that opens the file and reads it until it ends or fails, sending owner:

• {:gamepad_device, reader, :opened} once the file is open
• {:gamepad_device, reader, {:data, bytes}} for each js_event record read, its 8 bytes for Gamepad.Linux.decode/2
• {:gamepad_device, reader, {:closed, reason}} when the file could not be opened, ended (:eof) or failed, as when the controller is unplugged; the process then exits normally

Each read waits on one of the VM's dirty I/O threads until the controller sends something. A reader asked to exit while waiting goes once that read returns; its file closes with it.