cinder retype corrupts volume in some scenarios

When converting from NFS to CEPH retype will not convert a qcow source to raw before writing it to the destination volume. This might be applicable to other backends as well and results in a corrupt destination volume.

It will just read the qcow file and write it to ceph without converting it to raw. In this specific case it will use _copy_volume_with_file() with the source as a path and the destination as a RawIOBase so we could probably not use qemu-img convert on the fly in any way, but would have to convert it (to a temporary file) before the actual transfer of the data to the destination takes place. Another option could be to nbd mount the qcow - but that comes with it's own bunch of potential issues and requirements.