1) remove all running bridges.
----------------------------
# virsh net-destroy default
----------------------------


2) backup existing configs.
------------------------------------
# cd /etc/sysconfig/network-scripts/
# mkdir bak
# cp -p ifcfg-* bak
------------------------------------


3) virsh network interface commands start with 'iface-'
--------------------------
# virsh help | grep iface
--------------------------

4) The iface-dumpxml command will create an XML representation of the ifcfg-eth1 file. Take a look at it, and create a copy.

-----------------------------------------
# virsh iface-dumpxml eth1

# virsh iface-dumpxml eth1 > bak/eth1.xml
------------------------------------------

5) Copy the XML from bak/eth1.xml to br0.xml, and make modifications such that bridge should look as below:
----------------------------------------
[root@kobain bak]# cat eth1.xml 
<interface type='ethernet' name='eth1'>
  <start mode='onboot'/>
  <mac address='00:21:9B:73:E2:65'/>
  <protocol family='ipv4'>
    <dhcp/>
  </protocol>
</interface>
**********************************
[root@kobain bak]# cat br0.xml 
<interface type='bridge' name='br0'>
  <start mode='onboot'/>
  <protocol family='ipv4'>
    <dhcp/>
  </protocol>

  <bridge>
    <interface type='ethernet' name='eth1'>
      <mac address='00:21:9B:73:E2:65'/>
    </interface>
  </bridge>    
</interface>

---------------------------------------------

6)Define the bridge interface to libvirt. At this point there is an error message, but it may be ignored.

---------------------------
#virsh iface-define br0.xml
----------------------------

7) restart the network and do an "virsh iface-list"