engine: exception on run once when changed display protocol

Regression from: af23e740b091832d9f12dc47083fe89007ad9b87

The problem was that the VdsProperties.Device part of the video card was added
as VmDeviceType which was not possible to serialize.

Fixed by getting the String out of it the same way as it used to be.

Change-Id: I89127115b9925574a45e5d81a90bbad2c01e3d22
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>