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

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

<filesystem> devices lack the model= attribute which is used by
most other device types. To eventually support
virtio-9p-pci-{non-}traditional in qemu, let's add a standard
model= attribute. The accepted values are:

- virtio
- virtio-transitional
- virtio-non-transitional

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