Use packed structure instead of pointer arithmetics.

Also, merge two redundant traces into one.
Signed-off-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20171012095319.136610-5-vsementsov@virtuozzo.com>
[eblake: tweak and mention impact on traces, fix errp usage]
Signed-off-by: NEric Blake <eblake@redhat.com>