manual: Don't suggest exposing VM port to local network.

The setting

    QEMU_NET_OPTS="hostfwd=tcp::2222-:22"

caused the VM's port 2222 to be advertised on the host as
`0.0.0.0:2222`, thus anybody in the local network of the host
could SSH into the VM.
Instead, port-forward to localhost only.

Use `127.0.0.1` also on the VM side, otherwise connections to
services that, in the VM, bind to `127.0.0.1` only
(doing the safe approach) do not work.

See e.g. https://github.com/NixOS/nixpkgs/issues/100192
for more info why localhost listening is the best default.