Re-write the logic in qemu_fwcfg_list_firmware(), add a function qemu_fwcfg_read_firmware_list() to handle reading firmware list. Signed-off-by: Miao Yan <yanmiaobest@gmail.com> Reviewed-by: Bin Meng <bmeng.cn@gmail.com> Tested-by: Bin Meng <bmeng.cn@gmail.com>