x86: qemu: add fw_cfg support

The QEMU fw_cfg interface allows the guest to retrieve various data
information from QEMU. For example, APCI/SMBios tables, number of online
cpus, kernel data and command line, etc.

This patch adds support for QEMU fw_cfg interface.

Signed-off-by: Miao Yan <yanmiaobest@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>