I just did an upgrade to dev-python/pygtk-2.24.0. It has the new glade use flag. If turned off, virt-manager will not run correctly. The following error dialog is given: Error starting Virtual Machine Manager: No module named glade If pygtk is rebuild with the glade USE flag, then virt-manager launches correctly. Therefore I think that virt-manager needs to explicitely require pygtk[glade]
pygtk-2.24.0-r1 forces glade again