If the image file cannot be opened and was created as a temporary file,
it should be deleted; thus, in this case, we should jump to the
"unlink_and_fail" label and not just to "fail".

Reported-by: Benoît Canet <benoit@irqsave.net>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>