Currently (337), the "Launch viewer" button in Cockpit Machine will launch an external viewer application by letting the browser download a `console.vv` file that (hopefully) instructs the OS to launch the `remote-viewer` application. If the VM has VNC graphics, `remote-viewer` is instructed to use that. Otherwise, if the VM has SPICE graphics, it is instructed to use that.

People want to connect to SPICE even for machines that have VNC graphics.

Also, we will want to launch viewers directly via links. Doing this might or might not work, depending on how the OS is configured and Cockpit can't detect that reliably. (`vnc://` links need a very recent version of remote-viewer, for example.)

Also, remote-viewer might get support for SSH tunneling, giving us one more option.

So we should start with designing how the "Launch viewer" button can launch both VNC and SPICE, and keep in mind that the list will grow.