connect Open vSwitch switch to real network : iptables masquerade

The answer is not really mine, but... Try following Open vSwitch on VirtualBox

There was also a brief text file from some Fedora people on similar subject: libvirt and OpenVSwitch in a form of text file... wasn't able to locate it though.

as per comment, here's short summary:

(1) Create vnet0 interface and br0 bridge:

ovs-vsctl add-br br0
ip tuntap add mode tap vnet0
ip link set vnet0 up
ovs-vsctl add-port br0 vnet0
ip link # (View the created interface)

(2) spin up VM that uses vnet0 interface (as a bridged adapter)

(3) connect bridge br0 to real network:

ovs-vsctl add-br br0 
ovs-vsctl add-port br0 eth0 
ovs-vsctl add-port br0 vnet0 
ifconfig eth0  0 && ifconfig br0 192.168.1.(X) netmask 255.255.255.0
route add default gw 192.168.1.1 br0
route del default gw 192.168.1.1 eth0