When I have played with KVM using libvirtd/virsh, I disabled section in libvirt's xml configs for VMs an this gives me CPU usage from 7-8% in idle mode per VM to 1-2% for windows guests. This was a great advantage. But how to apply such trick when using Ganeti?
Sorry for disturbing the society, have found answer myself, when explored Ganeti's sources. If someone interested, just modify an instance:
When mouse_type is empty, it doesn't add "-usb" option to KVM. vnc_bind_address should be empty too, otherwise it adds -usbdevice tablet. CPU usage in idle mode changed from ~7% to ~1%.
I recommend you submit a bug on their tracker so they could possibly do that by default in a future release.