hw/arm/boot: Report error if there is no fw_cfg device in the machine

If the user provides both a BIOS/firmware image and also a guest
kernel filename, arm_setup_firmware_boot() will pass the
kernel image to the firmware via the fw_cfg device. However we
weren't checking whether there really was a fw_cfg device present,
and if there wasn't we would crash.

This crash can be provoked with a command line such as
 qemu-system-aarch64 -M raspi3 -kernel /dev/null -bios /dev/null -display none

It is currently only possible on the raspi3 machine, because unless
the machine sets info->firmware_loaded we won't call
arm_setup_firmware_boot(), and the only machines which set that are:
 * virt (has a fw-cfg device)
 * sbsa-ref (checks itself for kernel_filename && firmware_loaded)
 * raspi3 (crashes)

But this is an unfortunate beartrap to leave for future machine
model implementors, so we should handle this situation in boot.c.

Check in arm_setup_firmware_boot() whether the fw-cfg device exists
before trying to load files into it, and if it doesn't exist then
exit with a hopefully helpful error message.

Because we now handle this check in a machine-agnostic way, we
can remove the check from sbsa-ref.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/503
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20210726163351.32086-1-peter.maydell@linaro.org

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