virsh: Fix job status indicator for 0 length block jobs
    
    Although 0 length block jobs aren't entirely useful, the output of virsh
    blockjob is empty due to the condition that suppresses the output for
    migration jobs that did not start. Since the only place that actually
    uses the condition that suppresses the output is in migration, let's
    move the check there and thus add support for 0 of 0 equaling to 100%.
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1196711