Comment 6 for bug 1356633

The SPICE agent provides a transport channel for mouse events which is better than USB tablet as it avoids the high CPU burn that USB tablet suffers from. So when SPICE agent is enabled, avoiding USB tablet is desirable to cut the baseline CPU usage of VMs and thus increase overall host CPU capacity / VM density per host.

Of course the logic that Nova applies here is not 100% ideal - there will always be some portion of scenarios which may want to keep the USB tablet, but on balance disabling it is a win in general. We likely want to make it possible to override the default behaviour based on an image property, but keep USB disabled by default when SPICE agent is on.