qemu-img cannot repair a qcow2 in an LV because size is mis-detected when qcow2 is on an LV
Description of problem:
This is RHEV with Tb's of VMs which need to be repaired due to a datacenter-wide (the real datacenter) power outage.

Each of these VMs are on individual LVs but qemu-img check fails to perform repairs:


```
ERROR cluster 24481205 refcount=0 reference=1
ERROR cluster 24481206 refcount=0 reference=1
Rebuilding refcount structure
ERROR writing refblock: No space left on device <============
qemu-img: Check failed: No space left on device
```

Running qemu-img check or info on the LV (/dev/dm-*) works well but repairs cannot be completed:

```
# qemu-img info /dev/cdd4e215-8c6b-4877-b2be-fdba383e7eb0/fb32333b-2334-4e10-8c42-02bc97e826cc
image: /dev/cdd4e215-8c6b-4877-b2be-fdba383e7eb0/fb32333b-2334-4e10-8c42-02bc97e826cc
file format: qcow2
virtual size: 1.5 TiB (1649267441664 bytes)
disk size: 0 B <================================
cluster_size: 65536
Format specific information:
    compat: 1.1
    compression type: zlib
    lazy refcounts: true
    refcount bits: 16
    corrupt: false
    extended l2: false
```
Steps to reproduce:
1. Have a damaged VM with its qcow2 in an LV
2. run 'qemu-img check <device>' verify that it properly detects the blocks which need fixing.
3. run 'qemu-img check -r all <device>', it exits with 'no space left on device´ after a few seconds.
Additional information:
https://bugzilla.redhat.com/show_bug.cgi?id=1519071


Here is one example: