vm-configurator: Explicitly assign mouse

This is to ensure that we have a USB mouse in new domains. While we have
not received any reports of PS/2 mouse (added by libvirt/qemu) being
slower than USB mouse, we have had reports about that about PS/2
keyboard vs. USB one. While we are adding USB keyboard, lets add USB
mouse too, just to be sure.

https://bugzilla.gnome.org/show_bug.cgi?id=735768