​You can do that.
But I still think that setting to qxl/spice should work In first round without setting to cirrus/vnc first, since the "​

​

​

Graphics protocol" is currently set to "None​

​"​

 

​and not "spice" AFAIR (as displayed in general sub tab).

If it is set to "None", then after the settings in (2) you will be notified with a pop-up displaying the following:
"Changes that require Virtual Machine restart: graphicsProtocol"

and after a restart both devices should be added.

​