Thanks for your replies !
I've tested with the option 'Other OS'. It works with a couple of Win10 1803 virtual machines, but not all.
I'll investigate why.
But my first conclusions are :
Windows sucks ^_^.
Windows lies about the real CPU usage.
I've some Windows 10 1803 guests on my clusters. They're totally idle (according to the windows process manager) but on the host side they consume a lot of CPU, between 6% and 15%.
All the windows installations are fresh installations, with tablet disabled, ballon and qemu guest agent...