qemu: format server interface without a listen address
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1130390
    
    The listen address is not mandatory for <interface type='server'>
    but when it's not specified, we've been formatting it as:
    -netdev socket,listen=(null):5558,id=hostnet0
    which failed with:
    Device 'socket' could not be initialized
    
    Omit the address completely and only format the port in the listen
    attribute.
    
    Also fix the schema to allow specifying a model.