block/nbd-client: use traces instead of noisy error_report_err

block/nbd-client: use traces instead of noisy error_report_err

Reduce extra noise of nbd-client, change 083 correspondingly.

In various commits (be41c100 in 2.10, f140e300 in 2.11, 78a33ab
in 2.12), we added spots where qemu as an NBD client would report
problems communicating with the server to stderr, because there
was no where else to send the error to.  However, this is racy,
particularly since the most common source of these errors is when
either the client or the server abruptly hangs up, leaving one
coroutine to report the error only if it wins (or loses) the
race in attempting the read from the server before another
thread completes its cleanup of a protocol error that caused the
disconnect in the first place.  The race is also apparent in the
fact that differences in the flush behavior of the server can
alter the frequency of encountering the race in the client (see
commit 6d39db96).

Rather than polluting stderr, it's better to just trace these
situations, for use by developers debugging a flaky connection,
particularly since the real error that either triggers the abrupt
disconnection in the first place, or that results from the EIO
when a request can't receive a reply, DOES make it back to the
user in the normal Error propagation channels.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20181102151152.288399-4-vsementsov@virtuozzo.com>
[eblake: drop depedence on error hint, enhance commit message]
Signed-off-by: Eric Blake <eblake@redhat.com>