21786 scan devices incremental disappearing devices fix

This is fix for 21786. Issue was caused by always setting "incremental" parameter to false in vdsm. In this case vdc was removing devices and then adding again when scaning was executed. Fix is to set this parameter to true, so the devices will not be removed during scan. Please review, as I might be not fully aware of all consequences of this change.

I think it would be better to have this parameter from caller, not just set in the code. But this will require changes in couple of places, will take time and I am not sure if we really need that.