A problem I’ve run into several times occurs when trying to run virt-manager on Gnome. When connecting to a new server, the connection will fail the the error:
Unable to connect to libvirt.
You need to install openssh-askpass or similar to connect to this host.
To get past this, open a terminal and run the following command:
$ sudo virt-manager --no-fork
Then pay attention to the terminal window, as any password prompts will appear there.