Test guestfs without support for close_on_exit

A previous commit, fd621ef05a024b8319f2c67f7e5c084b9ab77755, fixed support for
versions of libguestfs that did not support the close_on_exit parameter.  This
patch adds unit test coverage for that code path.

Change-Id: Ic3cc0057e24ac00e6435a9555a19c076ddc9b02e
Related-Bug: #1271562
Related-Bug: #1261475
Merge "Test guestfs without support for close_on_exit"