Trace previous state, move tracepoint to runstate_set start (to cover
all cases for debugging), add string representations of traced states.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id: 20190124125154.474650-1-vsementsov@virtuozzo.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>