The console settings form on the VM detail page renders the VNC password
(PasswordInput(render_value=True)) and was shown to every is_staff user.
A staff user with the global view_instances permission, or a staff owner
without is_vnc, could read the VNC password of every VM they could see.
Add instances.utils.can_manage_console (active superuser, or an active
owner with is_change and is_vnc), which matches what update_console
already enforced. The detail view only builds the form, and so only puts
the password into the page, when it returns True; the template and
update_console use the same rule.
