rust: qom: add initial subset of methods on Object

rust: qom: add initial subset of methods on Object

Add an example of implementing instance methods and converting the
result back to a Rust type.  In this case the returned types are a
string (actually a Cow<str>; but that's transparent as long as it derefs
to &str) and a QOM class.

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