storage_backend: Resolve Coverity issue
storage_backend: Resolve Coverity issue

The switch statement in 'virStorageBackendCreateQemuImgOpts' used the
for loop end condition 'VIR_STORAGE_FILE_FEATURE_LAST' as a possible value,
but since that cannot happen Coverity spits out a DEADCODE message. Adding
the Coverity tag just removes the Coverity message

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