nbd: refactor tracing

Reorganize traces: move, reword, add information, drop extra ones.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20170707152918.23086-10-vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>