VirtIO-based input devices

The USB-based tablet is often the only USB device in an x86 instance, while the USB-based keyboard is often the only such device in an AArch64 instance (x86 have PS2 keyboards and mice). Replacing these with virtio-based devices can eliminate the need to have a USB host adapter in the instance. We can do this by adding a new 'hw_input_bus' image metadata property.