qemu: build command line for virtio-input-host device
<input type='passthrough' bus='virtio'>
<source evdev='/dev/input/event1234'/>
</input>
results in:
-device virtio-input-host-pci,id=input0,evdev=/dev/input/event1234
https://bugzilla.redhat.com/show_bug.cgi?id=1231114