virDomainGetBlockJobInfo: Fix corner case when qemu reports no info
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1372613
    
    Apparently, some management applications use the following code
    pattern when waiting for a block job to finish:
    
      while (1) {
        virDomainGetBlockJobInfo(dom, disk, info, flags);
    
        if (info.cur == info.end)
            break;
    
        sleep(1);
      }
    
    Problem with this approach is in its corner cases. In case of
    QEMU, libvirt merely pass what has been reported on the monitor.
    However, if the block job hasn't started yet, qemu reports cur ==
    end == 0 which tricks mgmt apps into thinking job is complete.
    
    The solution is to mangle cur/end values as described here [1].
    
    1: https://www.redhat.com/archives/libvir-list/2016-September/msg00017.html
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>