storage: rework qemu-img command line generation
storage: rework qemu-img command line generation

Split out option string generation to make adding new options easier
and simplify the code.

libvirt/libvirt,jardasgit/libvirt,taget/libvirt,nertpinx/libvirt,jfehlig/libvirt,agx/libvirt,fabianfreyer/libvirt,elmarco/libvirt,elmarco/libvirt,eskultety/libvirt,VenkatDatta/libvirt,zhlcindy/libvirt-1.1.4-maintain,zippy2/libvirt,shugaoye/libvirt,cbosdo/libvirt,taget/libvirt,cbosdo/libvirt,libvirt/libvirt,nertpinx/libvirt,datto/libvirt,andreabolognani/libvirt,shugaoye/libvirt,andreabolognani/libvirt,andreabolognani/libvirt,VenkatDatta/libvirt,VenkatDatta/libvirt,VenkatDatta/libvirt,zippy2/libvirt,shugaoye/libvirt,elmarco/libvirt,zhlcindy/libvirt-1.1.4-maintain,jfehlig/libvirt,olafhering/libvirt,jfehlig/libvirt,shugaoye/libvirt,eskultety/libvirt,crobinso/libvirt,rlaager/libvirt,olafhering/libvirt,fabianfreyer/libvirt,elmarco/libvirt,taget/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,cbosdo/libvirt,olafhering/libvirt,eskultety/libvirt,nertpinx/libvirt,nertpinx/libvirt,agx/libvirt,crobinso/libvirt,agx/libvirt,taget/libvirt,datto/libvirt,zippy2/libvirt,crobinso/libvirt,fabianfreyer/libvirt,cbosdo/libvirt,zippy2/libvirt,rlaager/libvirt,elmarco/libvirt,rlaager/libvirt,nertpinx/libvirt,cbosdo/libvirt,fabianfreyer/libvirt,rlaager/libvirt,datto/libvirt,zhlcindy/libvirt-1.1.4-maintain,VenkatDatta/libvirt,agx/libvirt,libvirt/libvirt,libvirt/libvirt,jardasgit/libvirt,taget/libvirt,jardasgit/libvirt,shugaoye/libvirt,crobinso/libvirt,zhlcindy/libvirt-1.1.4-maintain,datto/libvirt,eskultety/libvirt,olafhering/libvirt,jardasgit/libvirt,agx/libvirt,datto/libvirt,andreabolognani/libvirt,zhlcindy/libvirt-1.1.4-maintain,andreabolognani/libvirt,jfehlig/libvirt,eskultety/libvirt,rlaager/libvirt