HI, I was having the same problem in Oneiric 64 bit. I was using the cmd line to launch virt-manager using $ virt-manager. The issue was solved when: a. I had installed ssh-askpass into my system b. I started using GUI and not cmd line to launch virt-manager. When cmdlink is used we get some GTK warnings which may also be the cause of the issue.
Regs, Deepak