Comment 3 for bug 1620633

Created image can be checked without starting a VM

qemu-img check /var/lib/uvtool/libvirt/images/kvmguest-precise.qcow
qemu-img: '' uses a qcow2 feature which is not supported by this qemu version: QCOW version 3
qemu-img: Could not open '/var/lib/uvtool/libvirt/images/kvmguest-precise.qcow': Operation not supported

This even happens with a Compat 0.10 XML on creation:
<volume>
  <name>kvmguest-precise.qcow</name>
  <allocation>0</allocation>
  <capacity unit="G">8</capacity>
  <target>
    <format type="qcow2"/>
    <compat>0.10</compat>
  </target>
  <backingStore>
    <path>/var/lib/uvtool/libvirt/images/x-uvt-b64-Y29tLnVidW50dS5jbG91ZC5kYWlseTpzZXJ2ZXI6MTIuMDQ6YW1kNjQgMjAxNjA4Mjk=</path>
    <format type="qcow2"/>
  </backingStore>
</volume>

A qemu-img created image works
qemu-img create -f qcow2 /tmp/test.qcow2 10M
Formatting '/tmp/test.qcow2', fmt=qcow2 size=10485760 encryption=off cluster_size=65536

qemu-img check /tmp/test.qcow2
Leaked cluster 0 refcount=2 reference=1
Leaked cluster 1 refcount=2 reference=1
Leaked cluster 2 refcount=2 reference=1
Leaked cluster 3 refcount=2 reference=1

4 leaked clusters were found on the image.
This means waste of disk space, but no harm to data.