Description of problem:
rhel6 have no specified the backing file format after create snapshot when use qemu-img info to check it, but rhel7.0 has it.

Version-Release number of selected component (if applicable):
host info:
2.6.32-412.el6.x86_64
qemu-kvm-rhev-0.12.1.2-2.397.el6.x86_64

How reproducible:
100%

Steps to Reproduce:
1.for rhel6.5:
# qemu-img create -f qcow2 -b data-disk.qcow2 -F qcow2 sluo-snapshot
Formatting 'sluo-snapshot', fmt=qcow2 size=10737418240 backing_file='data-disk.qcow2' backing_fmt='qcow2' encryption=off cluster_size=65536 
# qemu-img info sluo-snapshot 
image: sluo-snapshot
file format: qcow2
virtual size: 10G (10737418240 bytes)
disk size: 196K
cluster_size: 65536
backing file: data-disk.qcow2
2.for rhel7.0:
# qemu-img create -f qcow2 -b data-disk.qcow2 -F qcow2 sluo-snapshot
Formatting 'sluo-snapshot', fmt=qcow2 size=1073741824 backing_file='data-disk.qcow2' backing_fmt='qcow2' encryption=off cluster_size=65536 lazy_refcounts=off 
# qemu-img info sluo-snapshot 
image: sluo-snapshot
file format: qcow2
virtual size: 1.0G (1073741824 bytes)
disk size: 196K
cluster_size: 65536
backing file: data-disk.qcow2
backing file format: qcow2           <------------------here

Actual results:

Expected results:
should add a item to specify the backing file format when use qemu-img info to check it.

Additional info: