core: Add TODO note about deprecated VM state

This patch adds a TODO comment that prevents us from forgetting to
delete deprecated states sent by VDSM. We should delete them as soon as
3.3 is no longer supported.

Change-Id: I3a5cce6b5337d4de3bf84092429f19fce5e0f574
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>