NFS image to volume uses raw format regardless of settings

While setting up a cluster using Debian Buster's OpenStack packages I noticed that the NFS backend for Cinder was refusing to honor the "nfs_qcow2_volumes" setting more often than it was honoring it.

It seems that this setting is ignored whenever a volume is created from an image, and in my usage this is the most common route for volume creation to take place.

The attached patch seems to fix the problem in Debian's python3-cinder package (version 2:13.0.3-1). I haven't tested with newer versions of Cinder, but the code looks more or less identical.