qapi: Pass '-u' when doing non-silent diff

qapi: Pass '-u' when doing non-silent diff

Ed-script diffs are awful compared to context diffs.  Fix another
'diff -q' while in the area (if the files are different, being
noisy makes it easier to diagnose why).

While at it, diff .err before .out, because if a test fails, .err
is more likely to contain the most important information for
fixing the failure.

Fixes: 46ec4fce
Signed-off-by: Eric Blake <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Message-Id: <20180315125116[email protected]>