acpi-build: properly decrement objects' reference counters

Object returned by object_property_get_qobject needs its reference counter to
be decremented when it is not needed by caller anymore.

Signed-off-by: Kirill Batuzov <batuzovk@ispras.ru>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>