[ubuntu] Redirecting selected ports to virtual machines

I am using Ubuntu 12.04 and have set up and qemu/kvm guest acording to this very good tutorial: https://help.ubuntu.com/community/KVM/

The client is installed (debian) and is working well. I can access the internet out of my guest and would now like to set up an web server.
For now I only have one IP adresse over my wlan device. Later I want to move the guest to another host which has several IPs and build a bridge.

So for now I have set up a "Usermode Networking" according to this: https://help.ubuntu.com/community/KVM/Networking (default mode).

Now I want to reach my guest webserver out of the internet/my lan. My host IP is 192.168.0.150 and my guest IP is 192.168.122.178.

If I open the web browser on my host I can reach the apache of the guest by entering "192.168.0.150".

So I thought I just have to set the Iptables rule for redirecting the port 80 and I can reach it! But it doesnt work :(

Code:

iptables -t nat -A PREROUTING -d 192.168.0.150 -p tcp --dport 80 -j DNAT --to-destination 192.168.122.178:80

Have I done anything wrong?