rust: qom: split ObjectType from ObjectImpl trait

rust: qom: split ObjectType from ObjectImpl trait

Define a separate trait for fields that also applies to classes that are
defined by C code.  This makes it possible to add metadata to core classes,
which has multiple uses:

- it makes it possible to access the parent struct's TYPE_* for types
  that are defined in Rust code, and to avoid repeating it in every subclass

- implementors of ObjectType will be allowed to implement the IsA<> trait and
  therefore to perform typesafe casts from one class to another.

- in the future, an ObjectType could be created with Foo::new() in a type-safe
  manner, without having to pass a TYPE_* constant.

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>