qcow2: Fix image creation regression

When checking for errors, commit db89119d compares with the wrong values,
failing image creation even when there was no error. Additionally, if an
error has occured, we can't preallocate the image (it's likely broken).

This unbreaks test 023 of qemu-iotests.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
