vm-configurator: Set 'USB' bus for input devices
vm-configurator: Set 'USB' bus for input devices

Lets explicitly set the bus of input devices to be USB as for some
devices libvirt/qemu will choose PS/2 by default and for keyboards (at
least), PS/2 is reported to be slower than USB.

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

vbenes/gnome-boxes,vbenes/gnome-boxes