when you launch Qemu from command line, I remember you've got options so as to tell how many network cards the virtual machine has, and what network mode it uses -- got to check the manpage.
I think there are at least 2 network modes:
Default mode: Qemu is answering DHCP/DNS requests and acts as a gateway as well. In short, Qemu sets up a virtual lan, and the virtual machine is NATed from this virtual lan to the network(s) available to the real machine. NATing is done in a way that prevents machines in the real lan to access the virtual one.
In another mode - which name I don't remember -, a network interface is created on the real machine (the host), it's called tun0, tun1, tun2... one for each virtual machine you have. Both these interfaces receive IP addresses. You can normally configure tun0 with ifconfig is you need special config, and you can use any tun interface in iptables, route, brctl and any other program.
So, if you only need to have samba working between the host and the virtual machine(s), you should only have to use the correct network mode, and have samba listening to any network (I belireve that's default behaviour). If you're runnning a firewall, you'll have to change its config to take the new virtual interface into account.
If you need your virual machines to be accessible from the real network, you'll need either to route from the virtual network to the real one, or to bridge the different networks together. That can be done with route or brctl exactly as if they were real interfaces, real networks and real machines.
Hope that helps...