netdev: Make netdev arguments fetchable, and implement for netdev-vport.

netdev: Make netdev arguments fetchable, and implement for netdev-vport.

This gives network device implementations the opportunity to fetch an
existing device's configuration and store it as their arguments, so that
netdev clients can find out how an existing device is configured.

So far netdev-vport is the only implementation that needs to use this.

The next commit will add use by clients.

Reviewed by Justin Pettit.