I had a problem installing a new virtual machine using Xen and virt-manager on CentOS 5.1.
I could not choose a network device as the drop-down list/menu was empty. From the bug at the following page:
I downloaded the connection.py.patch file and copied it to the server at:
patch -p0 < connection.py.patch
when I started virt-manager again I had more success with the network options dialog.