PPC: Provide zero SVR for -cpu e500mc and e5500

Even though our -cpu types for e500mc and e5500 are no real CPUs that
actually have version registers, a guest might still want to access
said version register and that has to succeed for a guest to be happy.

So let's expose a zero SVR value on E500_SVR SPR reads.

Signed-off-by: Alexander Graf <agraf@suse.de>
