object: add class property initializer

This callback is used to set default value in following patch "object:
add object_property_set_defaut_{bool,str,int,uint}()".

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20200110153039.1379601-7-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>