As long as we guarantee RPC struct layout stability, we might as
well also guarantee RPC enum value constancy.

* src/Makefile.am (r1, r2, PDWTAGS): Adjust rule to pick up named
and anonymous enums.
* src/remote_protocol-structs: Add enum values.
* src/qemu_protocol-structs: Likewise.
* src/virnetprotocol-structs: Likewise.