conf: Add <input model='virtio-{non-}transitional'/>

conf: Add <input model='virtio-{non-}transitional'/>

<input> devices lack the model= attribute which is used by
most other device types. To eventually support
virtio-input-host-pci-{non-}traditional in qemu, let's add
a standard model= attribute. This just adds the domain_conf
wiring

Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: Cole Robinson <crobinso@redhat.com>