qemu_fdt_add_path() works like qemu_fdt_add_subnode(), except it
also adds all missing subnodes from the given path. We'll use it
in a coming patch where we will add cpu-map to the device tree.

And we also tweak an error message of qemu_fdt_add_subnode().

Co-developed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Yanan Wang <wangyanan55@huawei.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Cc: David Gibson <david@gibson.dropbear.id.au>
Cc: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20211020142125.7516-3-wangyanan55@huawei.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>