http://6i4b2igvvpe53lp4vpm4frtjvdepiyfczdemljgomn5mqjtfl2w7scyd.onion/tag/networking
It makes QEMU create a virtual network, with the usual DHCP server and connection to the host, but without Internet access. In addition, it is possible to specify various guestfwd options. Sadly, the forward mode opens only one connection when the machine boots.