vt: add comment for unbind_con_driver()

- add comment for unbind_con_driver().
- bind_con_driver() is made private again

Signed-off-by: Jesse Barnes <jesse.barnes@intel.com>
Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>