qcow2: check_errors are fatal

qcow2: check_errors are fatal

When trying to repair a dirty image, qcow2_check() may apparently
succeed (no really fatal error occurred that would prevent the check
from continuing), but if check_errors in the result object is non-zero,
we cannot trust the image to be usable.

Reported-by: R. Nageswara Sastry <[email protected]>
Buglink: https://bugs.launchpad.net/qemu/+bug/1728639
Signed-off-by: Max Reitz <[email protected]>
Message-id: 20171110203111[email protected]
Reviewed-by: Eric Blake <[email protected]>
Signed-off-by: Max Reitz <[email protected]>