Refactor XenAgent

Refactor XenAgent

- Merge IDeviceCreator interface into CDeviceList base class
- Add CXenIfaceDeviceList and CConvDeviceList, derived from CDeviceList
- Moves IFace functionality to CXenIfaceDeviceList
- Moves Conv device functionality to CConvDeviceList
- Seperate RegisterDeviceNotification from SetupApi enumeration, so that static
  devices do not need to hold the handle open (i.e. Conv device)
- CConvDeviceList only opens the handle when neccessary, as the MS driver for
  the ACPI device only allows 1 open handle at a time.

Signed-off-by: Owen Smith <owen.smith@citrix.com>