v1 was here:
https://www.redhat.com/archives/libguestfs/2018-August/thread.html#00287
v2:
- The -oa option now gives an error; apparently Cinder cannot
generally control sparse/preallocated behaviour, although certain
Cinder backends can.
- The -os option maps to Cinder volume type; suggested by Matt Booth.
- Add a simple test.