qemu: Avoid using stale data in virDomainGetBlockInfo
    
    CVE-2013-6458
    
    Generally, every API that is going to begin a job should do that before
    fetching data from vm->def. However, qemuDomainGetBlockInfo does not
    know whether it will have to start a job or not before checking vm->def.
    To avoid using disk alias that might have been freed while we were
    waiting for a job, we use its copy. In case the disk was removed in the
    meantime, we will fail with "cannot find statistics for device '...'"
    error message.