hw/pci-host/raven: Propagate error in raven_realize()

When an Error** reference is available, it is better to
propagate local errors, rather then using generic ones,
which might terminate the whole QEMU process.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Message-Id: <20231120213301.24349-26-philmd@linaro.org>