Start work on XENFILT

Start to move towards a new EMULATED interface that will allow child
drivers to check presence of 'DEVICE's (by DeviceID and InstanceID) and
'DISK's (by Controller, Target, Lun tuples). This first commit just puts
in the groundwork for setting the right data on the PDOs.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>