Expanding on Justin's idea
I'm no genius, but this worked for me and you somewhat need to know what you are doing. host:Ubuntu 12.04. guest: Debian Testing (Jessie)
For Debian based systems configure the network interface by adding or editing the following in the host /etc/network/interfaces file (ifconfig -a is your friend):
iface br0 inet static
Restart networking on the host machine or just restart the computer.
Then using virt-manager added the unbridged network device br0 to the network interface of the guest.
Then on the guest system you will have to assign a static IP address since there is no DHCP running on the br0 device. Setting a static IP address will depend on the OS.
For a Debian based guest you would edit /etc/network/interfaces and add:
iface eth0 inet static
I very much needed this setup and spent a couple of nights trying to find this through trial and error. You might need to change the numbers on the devices (br1 instead of br0, or eth1 instead of eth0). If your guest needs Internet access, then more setup will be needed.