object: avoid extra class property key duplication

Like object properties, no need to duplicate property name, as it is
owned already by ObjectProperty value.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20200110153039.1379601-6-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>