Rework the classes a bit to make MockDevices possible

This adds support for creating Mock devices that return pre-cooked datain to the SCSI command/unmarshalling.

Add a partial "unmarshall known fixed Standard page" to show the mock device in action.
This makes unit testing for the command unmarshalling a lot easier.


Signed-off-by: Ronnie Sahlberg <ronniesahlberg@gmail.com>