rust: qom: move ClassInitImpl to the instance side

rust: qom: move ClassInitImpl to the instance side

Put all traits on the instance struct, which makes it possible to reuse
class structs if no new virtual methods or class fields are added.
This is almost always the case for devices (because they are leaf
classes), which is the primary use case for Rust.

This is also simpler: soon we will find the implemented methods without
macros, and this removes the need to go from the class struct to the
instance struct to find the implementation of the *Impl traits.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>