tests: qemumonitorjson: Fix 'relative' node name detection test case
    
    Add the blockstats data and fix the expected output.
    
    Test data was created as:
    
    qemu-img create -f qcow2 img0 10M
    qemu-img create -f qcow2 -o "backing_fmt=qcow2,backing_file=img0" img1
    qemu-img create -f qcow2 -o "backing_fmt=qcow2,backing_file=img1" img2
    ...
    
    Reviewed-by: Eric Blake <eblake@redhat.com>