In various of cases, when trying to launch applications such as wireshark, virt-manager or similar from the VNC, you get an error similar to this:
** (virt-manager:5488): WARNING **: Could not open X display
Gtk couldn't be initialized
To solve it:
1. Make sure you are connected to the server with ssh and "-X" option
ssh root@my-server -X
2. In other cases, it means that the DISPLAY parameter is not set properly to be the VNC window.
# echo $DISPLAY
To set the DISPLAY parameter run (for example):
# export DISPLAY=i-vnc01:2
Now you should be good to launch your application.