Improve logging when python-guestfs/libguestfs isn't working

Bumped from debug to warn level, as most often refers to
missing python-guestfs package or other libguestfs misconfiguration,
and the fallback path is usually not desired.

Closes-bug: 1240339
Change-Id: I5d90a3ea109e79fe17221c001f94d49d599048ba
Merge "Improve logging when python-guestfs/libguestfs isn't working"