Sometimes virt-manager (over a less than optimal link) will be unusable. Never fear, it’s simply creating an ssh tunnel, and then connecting you to vnc. No reason to let it have all the fun.
First find out what port the vnc server you want to talk to is listening on
root@vm.foo.com:~# virsh dumpxml dns | grep graph
Connecting to uri: qemu:///system
There we see it’s port 5904, now tunnel that port over ssh
ssh -L 5904:localhost:5904 root@vm.foo.com
Then open the vnc session in a viewer
npowell@malaise:~$ vncviewer localhost::5904
Cibatta bing. You in!
