raid: conditionally reject convert to striped/raid0*

raid: conditionally reject convert to striped/raid0*

If SubLVs to be removed still exist after an image removing
conversion (i.e. "lvconvert --yes --force --stripes N "
with N < total stripes) any request to convert to a different
striped/raid* level has to be rejected until after those freed
SubLVs got removed by running the aforementioned lvconvert again.

Add tests to check conversion to striped/raid* gets rejected.
Enhance a test comment.

Related: rhbz1191935
Related: rhbz1366296