qemu-img.commit test correction

1. Correct the file name.
   The original image is what is generally called the backing file,
   whereas the new image you created is an overlay.

2. Remove the step which boot up the backing file before commit.
   It is not allowed to boot up the backing file before commit.
   If you have boot up, you have written to the backing file,
   which is likely to destory data on the overlays.

Signed-off-by: Cong Li <coli@redhat.com>