If you’ve installed Ubuntu 16.04 (in a virtual machine in my case) recently, you’ll find that if you launch Virt-Manager (for controlling KVM hypervisors) you’ll receive an error indicating the you need to install openssh-askpass or similar for Ubuntu. Unfortunately it has a different package name in Ubuntu (as Virt-Manager is a native RedHat product). To install the proper packages in UBuntu 16.04, run:
sudo apt-get install ssh-askpass ssh-askpass-gnome
And you should be able to launch normally. I also recommend using copied SSH keys to make the connection set-it-and-forget-it (til you change the root passwords that is).