storage: Use virStoragePoolObjGetDef accessor for RBD backend
storage: Use virStoragePoolObjGetDef accessor for RBD backend

In preparation for privatizing the object, use the accessor.

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