Do not try to detach non-existant loopback devices
Do not try to detach non-existant loopback devices

A user reported symptoms where the losetup line used to detach the
loopback device was failing in tar mode. We don't need to detach a
device that does not exist.

Change-Id: I807996e16199288927b49b4f300ae9b461cb8fe7
Closes-Bug: #1378033

zioproto/diskimage-builder,switch-ch/diskimage-builder,zioproto/diskimage-builder,openstack/diskimage-builder,varunarya10/diskimage-builder,switch-ch/diskimage-builder,takahashinobuyuki/diskimage-builder,rdo-management/diskimage-builder,takahashinobuyuki/diskimage-builder,openstack/diskimage-builder,eantyshev/diskimage-builder,NaohiroTamura/diskimage-builder,JioCloud/diskimage-builder,NaohiroTamura/diskimage-builder,rdo-management/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,JioCloud/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,eantyshev/diskimage-builder,varunarya10/diskimage-builder