Description of problem:
------------------------
after stream the entire chain, backing file still exists in info block output.


Version-Release number of selected component (if applicable):
--------------------------------------------------------------
qemu-kvm-0.12.1.2-2.262.el6rhev.x86_64


How reproducible:
-----------------
100%


Steps to Reproduce:
--------------------
1.(qemu) info block
drive-virtio-disk0: removable=0 io-status=ok file=/root/RHEL-Server-6.3-64-virtio.qcow2 ro=0 drv=qcow2 encrypted=0

2.(qemu) snapshot_blkdev drive-virtio-disk0 /root/sn1 qcow2
Formatting '/root/sn1', fmt=qcow2 size=21474836480 backing_file='/root/RHEL-Server-6.3-64-virtio.qcow2' backing_fmt='qcow2' encryption=off cluster_size=65536 
(qemu) info block
drive-virtio-disk0: removable=0 io-status=ok file=/root/sn1 backing_file=/root/RHEL-Server-6.3-64-virtio.qcow2 ro=0 drv=qcow2 encrypted=0

3.(qemu) block_stream drive-virtio-disk0 

4.(qemu) info block-jobs 
No active jobs
(qemu) info block
drive-virtio-disk0: removable=0 io-status=ok file=/root/sn1 backing_file=/root/RHEL-Server-6.3-64-virtio.qcow2 ro=0 drv=qcow2 encrypted=0

5. quit qemu-kvm.

6.[root@shu ~]# qemu-img info sn1
image: sn1
file format: qcow2
virtual size: 20G (21474836480 bytes)
disk size: 20G
cluster_size: 65536


Expected results:
------------------
After block stream finishes, set backing file as null.