hw/qdev: introduce qdev_is_realized() helper

Add a helper function to check whether the device is realized without
requiring the Big QEMU Lock. The next patch adds a second caller. The
goal is to avoid spreading DeviceState field accesses throughout the
code.

Suggested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20230516190238.8401-3-stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>