[Feature] Add ability to have internal networks like VirtualBox

Hi all, I'm wondering if it is possible to have an "Internal Network" configuration in UTM like VirtualBox

> Internal Networking is similar to bridged networking in that the VM can directly communicate with the outside world. However, the outside world is limited to other VMs on the same host which connect to the same internal network.

Ref: [Internal Network VirtualBox](https://docs.oracle.com/en/virtualization/virtualbox/6.0/user/network_internal.html)

I have tried using "Host Only" mode both with "Isolate Guest from host" turned on and off still I'm not able to achieve the functionality I want.

Context: I'm trying to run pfSense (emulated) and some client VM's and I want to setup a blank network so that my VM's will get the network configuration from pfSense and not UTM's/QEMU's/host's DHCP server.

*Note I have already tried passing direct QEMU arguments like `-device virtio-net-pci,netdev=net0 -netdev vmnet-host,id=net0` but that still seems to get either no IP or IP from my host's DHCP server.

I think this might possible as UTM is based on QEMU as I was able to find [How to create "internal network" using libvirt/QEMU/KVM stack](https://unix.stackexchange.com/questions/721794/how-to-create-internal-network-using-libvirt-qemu-kvm-stack).