Cannot run virt-manager inside a VNC session

Issue

Running virt-manager in a VNC session gives the following error message:

Unable to open connection to hypervisor URI 'qemu:///system':  
authentication failed  
Traceback (most recent call last):  
File "/usr/share/virt-manager/virtManager/connection.py", line 992, in _try_open [None], flags)  
File "/usr/lib64/python2.6/site-packages/libvirt.py", line 111, in openAuth  
    if ret is None:raise libvirtError('virConnectOpenAuth() failed')  
libvirtError: authentication failed