Support for equivalent to slirp guestfwd

[Peter Foley <pefoley@google.com>]

Hi,

I'm investigating migrating Google-internal uses of qemu's slirp-based
networking to passt.
One major gap I've discovered is the apparent lack of support in passt for
something like the "guestfwd" flag the slirp network backend handles:
https://www.qemu.org/docs/master/system/invocation.html
This allows us to open outbound connections from the VM to an external
service listening on a known port.

As far as I can tell, passt's tcp-ports and udp-ports flags appear to map
to slirp's hostfwd flags, only allowing traffic to flow into the VM.

Am I missing something? If this functionality indeed isn't supported, are
there plans to add it?

Thanks,

Peter Foley