contrib: add vhost-user-input

Add a vhost-user input backend example, based on virtio-input-host
device. It takes an evdev path as argument, and can be associated with
a vhost-user-input device via a UNIX socket:

$ vhost-user-input -p /dev/input/eventX -s /tmp/vui.sock

$ qemu ... -chardev socket,id=vuic,path=/tmp/vui.sock
  -device vhost-user-input-pci,chardev=vuic

This example is intentionally not included in $TOOLS, and not
installed by default.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20190514104126.6294-4-marcandre.lureau@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>