At least when two "relative" pointing devices are present at the same
time, like a ps2 mouse and usb mouse, I've seen major breakage caused
by host mouse events being used for each such device in virt-viewer,
effectively doubling all mouse events inside the guest (i.e. one click
=> double click, mouse movements are doubled, etc). Hence it feels
safer to disable the ps2 mouse and only rely on the ("absolute")
tablet device (which has further improvements via Spice).

Unfortunately it seems impossible to remove the ps2 mouse since it's
part of all QEMU machine templates, including the one we use,
presumably with the exception of 'none', which should be an empty
machine. However, I've so far not been able to come up with a config
that works based on 'none' (generally I get alias errors with the PCI
controller). So we also have to blacklist the module.

Will-fix: #9425