Always set errp on failure. The generic bdrv_open_driver supports
driver functions which can return a negative value but forget to set
errp.  That's a strange thing. Let's improve bdrv_qed_do_open to not
behave this way. This allows the simplification of code in
bdrv_qed_co_invalidate_cache().

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Message-Id: <20210202124956.63146-14-vsementsov@virtuozzo.com>
[eblake: commit message grammar tweak]
Signed-off-by: Eric Blake <eblake@redhat.com>