rust: qom: move bridge for TypeInfo functions out of pl011

rust: qom: move bridge for TypeInfo functions out of pl011

Allow the ObjectImpl trait to expose Rust functions that avoid raw
pointers (though INSTANCE_INIT for example is still unsafe).
ObjectImpl::TYPE_INFO adds thunks around the functions in
ObjectImpl.

While at it, document `TypeInfo`.

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