cg: correctly detect co-mounted controller mount point

Before we would not detect the mount point for co-mounted controllers. Things
still worked because we'd just re-mount them ourselves and traverse our own
mount point, but this saves an extra mount().

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>