qemu: format server interface without a listen address

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.