Description of problem:
------------------------
After block stream, qcow2 image disk size reaches to virtual size.


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

3.(qemu) block_stream drive-virtio-disk0

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

5.[root@shu ~]# qemu-img info RHEL-Server-6.3-64-virtio.qcow2 
image: RHEL-Server-6.3-64-virtio.qcow2
file format: qcow2
virtual size: 20G (21474836480 bytes)
disk size: 4.5G
cluster_size: 65536

  
Expected results:
------------------
Hi, i am not sure whether this is a bug, if not, please explain a little bit, and i highly recommend this as a RFE if can not be solved in 6.3, for streaming is mainly used by customer to update or re-arrange their storage, they probably don't want to see that after migrating to a larger storage, there is still no space left.