qemu.tests: check backingfile after stream job done

New test to verify image file no backingfile after stream done;
steps:
1. Boot guest, then create live snapshot file via monitor
2. Start block-stream job, the wait for job done
3. Check image file no backing-file by command "qemu-img info <image file>" and "info block" in monitors
4. Check the opening image file is expected image, and verify guest is live; 
Thanks,
Xu