softmmu: Provide a clue as to why device tree loading failed

softmmu: Provide a clue as to why device tree loading failed

fdt_open_into() obligingly returns an error code in case the operation
failed. So be obliging as well and use it in the error message.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20220116114649.40859-1-shentey@gmail.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>