Add a new virtio-input device, which connects to a vhost-user
backend.

Instead of reading configuration directly from an input device /
evdev (like virtio-input-host), it reads it over vhost-user protocol
with {SET,GET}_CONFIG messages. The vhost-user-backend handles the
queues & events setup.
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20190503130034.24916-5-marcandre.lureau@redhat.com

[ kraxel: drop -{non-,}transitional variants ]
[ kraxel: fix "make check" on !linux ]
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>