Deal with old versions of libguestfs

Commit 02abbef960b12deaa7a911788dbc56f3a0bd555a added the use of the
close_on_exit parameter.  This is supported by libguestfs 1.20 and up.
Catch the exeption raised when this parameter isn't supported and retry
without it.

Change-Id: Ic47c1cb997ece3710f8c4465733148fcfa94068d
Closes-Bug: #1271562
Related-Bug: #1261475
Merge "Deal with old versions of libguestfs"