docs: Explain how to debug libvirt initialization problems (RHBZ#680031).

diff --git a/virt-top/virt-top.pod b/virt-top/virt-top.pod
index 225c5ea..63aa30a 100755 (executable)
--- a/virt-top/virt-top.pod
+++ b/virt-top/virt-top.pod
@@ -424,6 +424,20 @@ will probably see a steady "background noise" of RX packets even when
 the network interface is idle or down.  These are caused by STP
 packets generated by the bridge.
 
+=head2 DEBUGGING LIBVIRT ISSUES
+
+virt-top tries to turn libvirt errors into informative messages.
+However if libvirt initialization fails then this is not possible.
+Instead you will get an obscure error like:
+
+ libvir: error : Unknown failure
+ Fatal error: exception Libvirt.Virterror(...)
+
+To see the cause of libvirt errors in more detail, enable libvirt
+debugging by setting this environment variable:
+
+ export LIBVIRT_DEBUG=1
+
 =head1 SEE ALSO
 
 L<top(1)>,