First install needed packages:
sudo apt-get install qemu-kvm libvirt-bin bridge-utils
And configure a network bridge in /etc/network/interfaces. Here is my complete file:
auto lo
iface lo inet loopback
allow-hotplug eth0
auto eth0
iface eth0 inet manual
auto br0
iface br0 inet dhcp
bridge_ports eth0
bridge_stp on
bridge_maxwait 0
bridge_fd 0