Add haproxy X-Forwarded-Host request header
Solution 1:
As Joel E Salas has mentioned via a comment, changing the Host header is a slight strange thing to do, but I suppose if the backends only understand certain values it could make sense.
In any event, you can just use the req.hdr fetch sample as a variable to http-request, thusly:
http-request set-header X-Forwarded-Host %[req.hdr(Host)]
You'll have to run that before you run the http-request set-header that rewrites the Host header, but it should work.