<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: NAndrea Bolognani <abologna@redhat.com>
Signed-off-by: NCole Robinson <crobinso@redhat.com>