Bug 1235406

Description of problem:

When using virt-Manager to create a new virtual disk that which uses another disk as a backing store, it fails to detect the correct type of the backing store causing the created disk to not work as intended (i.e. it doesn't use the data in the backing store because it can not read the backing store).

Version-Release number of selected component (if applicable): 1.2.1

How reproducible: Always

Steps to Reproduce:
1. Create a disk with virt-manager and set the type to "qcow2"
2. (Optional, but very useful) Install a version of Linux on that disk. This will serve as a test to tell whether or not the backing_store operation was successful.
3. Create another new disk with virt-manager set the type to "qcow2" and set the backing_store to the first disk you created.
4. If you installed Linux on the first disk, attempt to boot it using the second. This *should* work since the second is a COW copy of the first. It will *not* work.
5. Use "qemu-img info <path to second disk>" to inspect the second disk. You will see something like the following

````
image: test.qcow2
file format: qcow2
virtual size: 8.0G (8589934592 bytes)
disk size: 196K
cluster_size: 65536
backing file: <path to first disk>.qcow2
backing file format: raw
Format specific information:
    compat: 1.1
    lazy refcounts: true
    refcount bits: 16
    corrupt: false
````

Notice that the "backing file format" is set to "raw" not "qcow2".

6. To confirm this is really the issue, create a new "qcow2" disk with "qemu-img" directly using the following command

"# qemu-img create -f qcow2 -o backing_file=<path to first disk>,backing_fmt=qcow2 grading.qcow2 100G"

7. Run "qemu-img info <path to new disk>" and you will see something like the following,

````
image: grading.qcow2
file format: qcow2
virtual size: 100G (107374182400 bytes)
disk size: 500M
cluster_size: 65536
backing file: <path to first disk>.qcow2
backing file format: qcow2
Format specific information:
    compat: 1.1
    lazy refcounts: false
    refcount bits: 16
    corrupt: false
````

Notice how "backing file format" is now correctly "qcow2"

8. If you installed Linux on the first disk, create a new VM on virt-manager, tell it to use the new (third) disk you just created using the "qemu-img create" command. This new VM will now boot and work normally.


Actual results: virt-manager fails to detect format of the backing store when creating a new disk with a backing store. This causes the backing_store to be effectively ignored (strangely there are no explicit errors...it just acts like the other disk is empty)

Expected results: virt-manager should detect the format of the backing store and set that appropriately when creating a disk with a backing store.


Additional info: 
qemu-img version 2.3.0

I have only tested this with qcow2 disks. I can not say how this works with other disk types. I can only say that when you try to use qcow2 disks with a backing store in virt-manager the results are a broken disk.