this also rename the zio class "zio" to "zio-char-devices" to make
the sysfs structure more readable.

On some old version of udev it does not work; it set read/write
permission on every char device.

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>
Acked-by: Alessandro Rubini <rubini@gnudd.com>