storage: Use virStoragePoolObjGetDef accessor for iSCSI backend
storage: Use virStoragePoolObjGetDef accessor for iSCSI backend

In preparation for privatizing the object, use the accessor.

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