Call OnDeviceRemoved callback(s) during DeviceList.Stop()

Call OnDeviceRemoved callback(s) during DeviceList.Stop()

OnDeviceAdded is called for every pre-attached device during
DeviceList.Start(), the corresponding call to OnDeviceRemoved should be
called for each attached device during the DeviceList.Stop() call.
By calling the OnDeviceRemoved callback during the service stop, the
xenagent service can remove the "feature-[poweroff|reboot|s3|s4]" flags.

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