[CentOS] unable to get domain status from libvirt & KVM

I have a python script that monitors the VMs on physical host servers 
running Xen, but the script doesn't work properly on a server I just 
built with KVM.  The script runs as a non-root user and simply gathers 
some details on the status and names of the domains running on the host.

Both Xen and KVM servers are running the same version of libvirt 
(libvirt-0.6.3-20.1.el5_4) and have the same, default 
/etc/libvirt/libvirtd.conf config file.

To troubleshoot, I've been running python interactively.  Here's how my 
Xen servers behave:

$ python
Python 2.4.3 (#1, Sep  3 2009, 15:37:37)
[GCC 4.1.2 20080704 (Red Hat 4.1.2-46)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
 >>> import libvirt
 >>> conn = libvirt.openReadOnly(None)
 >>> domains = conn.listDomainsID()
 >>> print domains
[0, 3, 15, 16, 21, 24, 26, 30, 32, 36, 38, 41, 43, 45, 47, 49, 51, 55, 
63, 67]
 >>>

When I try the same thing on the KVM server:

$ python
Python 2.4.3 (#1, Sep  3 2009, 15:37:37)
[GCC 4.1.2 20080704 (Red Hat 4.1.2-46)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
 >>> import libvirt
 >>> conn = libvirt.openReadOnly(None)
14:33:07.303: error : No vport operation path found for host0
14:33:07.320: error : No vport operation path found for host4
14:33:07.325: error : No vport operation path found for host3
14:33:07.367: error : No vport operation path found for host1
14:33:07.368: error : No vport operation path found for host2
 >>> domains = conn.listDomainsID()
 >>> print domains
[]
 >>>

(The vport stuff is weird, but I found this posting that suggests its 
harmless 
(http://www.mail-archive.com/libvir-list@redhat.com/msg17477.html) so 
I'm ignoring it.)

However, when logged in as root on the KVM server, it works just like my 
Xen servers:

# python
Python 2.4.3 (#1, Sep  3 2009, 15:37:37)
[GCC 4.1.2 20080704 (Red Hat 4.1.2-46)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
 >>> import libvirt
 >>> conn = libvirt.openReadOnly(None)
 >>> domains = conn.listDomainsID()
 >>> print domains
[1]

Again, on the KVM server, plain old "virsh list" with the debug level 
set to "2"

$ export LIBVIRT_DEBUG=2
$ virsh list
14:21:06.532: error : No vport operation path found for host0
14:21:06.550: error : No vport operation path found for host4
14:21:06.555: error : No vport operation path found for host3
14:21:06.598: error : No vport operation path found for host1
14:21:06.599: error : No vport operation path found for host2
14:21:06.615: info : No security driver available
  Id Name                 State
----------------------------------

Permissions in /var/run/libvirt:

# ls -ld /var/run/libvirt/*
srwx------ 1 root root    0 Feb  5 08:53 /var/run/libvirt/libvirt-sock
srwxrwxrwx 1 root root    0 Feb  5 08:53 /var/run/libvirt/libvirt-sock-ro
drwxr-xr-x 2 root root 4096 Jan 21 14:38 /var/run/libvirt/network
drwxr-xr-x 2 root root 4096 Oct 20 18:50 /var/run/libvirt/qemu

Can someone provide some tips on what else I can check, if this might be 
a bug, or point out any mistakes that I might've made?  Any help is 
appreciated.

Thanks,
Tom