storage: Use virStoragePoolObjGetDef accessor for SCSI backend
storage: Use virStoragePoolObjGetDef accessor for SCSI backend

In preparation for privatizing the object, use the accessor.

fabianfreyer/libvirt,zippy2/libvirt,libvirt/libvirt,eskultety/libvirt,eskultety/libvirt,crobinso/libvirt,eskultety/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,nertpinx/libvirt,eskultety/libvirt,olafhering/libvirt,eskultety/libvirt,libvirt/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,zippy2/libvirt,olafhering/libvirt,nertpinx/libvirt,andreabolognani/libvirt,jfehlig/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,nertpinx/libvirt,libvirt/libvirt,crobinso/libvirt,fabianfreyer/libvirt,crobinso/libvirt,olafhering/libvirt,libvirt/libvirt,zippy2/libvirt,andreabolognani/libvirt,andreabolognani/libvirt,jardasgit/libvirt,jfehlig/libvirt,jfehlig/libvirt,jfehlig/libvirt,crobinso/libvirt,jardasgit/libvirt,zippy2/libvirt,nertpinx/libvirt,olafhering/libvirt,jardasgit/libvirt,nertpinx/libvirt