Correct DIB long option parsing
Correct DIB long option parsing

Fixes issues with option parsing. The --min-tmpfs and --image-cache
options where previously broken.

Closes-bug: 1282077
Change-Id: I40c62b16d854335902d1b6b5ceab7f0e1992623a

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