When you run virtual machines, the most convenient network setup is having a bridge between the virtual machine and your normal ethernet network. In Debian, I use the following config in /etc/network/interfaces to have such a bridge: 

# /etc/network/interfaces -- configuration file for ifup(8), ifdown(8)
 
# The loopback interface
auto lo
iface lo inet loopback
 
# The first network card - this entry was created during the Debian installation
# (network, broadcast and gateway are optional)
auto eth1
iface eth1 inet manual
 
auto br0
iface br0 inet static
    address 192.168.1.101
    network 192.168.1.0
    broadcast 192.168.1.255
    netmask 255.255.255.0
    gateway 192.168.1.100
    bridge_ports eth1
    bridge_fd 9
    bridge_hello 2
    bridge_maxage 12
    bridge_stp off

In this setup, the br0 gets the IP address that eth1 had before. Intuitively, I would have created a bridge which would connect eth1 and new interfaces, but when I did that, I could not get it to work.

I then used a command similar to this to install the virtual machine: 

virt-install --connect qemu:///system -n vm10 -r 512 --vcpus=2 -f ~/vm10.qcow2 -s 12 -c ~/debian-500-amd64-netinst.iso --vnc --noautoconsole --os linux --os-variant debianLenny --accelerate --network=bridge:br0 --hvm

You can adjust where necessary. What’s important, is the –network=bridge:br0. This makes sure that kvm is run with: 

kvm [more options] -net nic,macaddr=54:52:00:52:1c:7c,vlan=0,model=virtio -net tap,fd=7,script=,vlan=0,ifname=vnet0

The network setup looks like: 

br0       Link encap:Ethernet  HWaddr 00:15:17:23:83:67
          inet addr:192.168.1.101  Bcast:192.168.1.255  Mask:255.255.255.0
          inet6 addr: fe80::215:17ff:fe23:8367/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:1866633 errors:0 dropped:0 overruns:0 frame:0
          TX packets:1678602 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:892540330 (851.1 MiB)  TX bytes:971936093 (926.9 MiB)
 
eth1      Link encap:Ethernet  HWaddr 00:15:17:23:83:67
          inet6 addr: fe80::215:17ff:fe23:8367/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:13371212 errors:0 dropped:0 overruns:0 frame:0
          TX packets:12703312 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:100
          RX bytes:4234365756 (3.9 GiB)  TX bytes:201738186 (192.3 MiB)
          Memory:88180000-881a0000
 
lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:2672817 errors:0 dropped:0 overruns:0 frame:0
          TX packets:2672817 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:493599313 (470.7 MiB)  TX bytes:493599313 (470.7 MiB)
 
vnet0     Link encap:Ethernet  HWaddr 00:ff:2d:a0:76:34
          inet6 addr: fe80::2ff:2dff:fea0:7634/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:159072 errors:0 dropped:0 overruns:0 frame:0
          TX packets:310906 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:500
          RX bytes:10860687 (10.3 MiB)  TX bytes:465526222 (443.9 MiB)

source for all this.