$ cat /etc/network/interfaces
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
#auto em1
#iface em1 inet dhcp

# Bridged network
auto br0
iface br0 inet dhcp
    bridge_ports em1
    bridge_stp on
    bridge_fd 0
    bridge_maxwait 0
$ ifconfig
br0       Link encap:Ethernet  HWaddr f4:ce:46:b0:1e:f0
          inet addr:192.168.17.17  Bcast:192.168.17.255  Mask:255.255.255.0
          inet6 addr: fe80::f6ce:46ff:feb0:1ef0/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:338237 errors:0 dropped:0 overruns:0 frame:0
          TX packets:180207 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:460007814 (460.0 MB)  TX bytes:13900357 (13.9 MB)

em1       Link encap:Ethernet  HWaddr f4:ce:46:b0:1e:f0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:338311 errors:0 dropped:0 overruns:0 frame:0
          TX packets:181776 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:464748748 (464.7 MB)  TX bytes:13985821 (13.9 MB)
          Memory:c0400000-c0420000

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:65536  Metric:1
          RX packets:16 errors:0 dropped:0 overruns:0 frame:0
          TX packets:16 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:1296 (1.2 KB)  TX bytes:1296 (1.2 KB)

$ uvt-kvm create secondtest --bridge br0
$ uvt-kvm wait secondtest --insecure
uvt-kvm: error: libvirt domain 'secondtest' has no NIC MACs available.