ZiocharDevice.py: make fdc and fdd private

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>
ZioInterface.py: add object attribute lastctrl

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>
ZioInterface.py: read_block() read data and contrl by default

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>