hw/riscv: Add fw_cfg support to virt

hw/riscv: Add fw_cfg support to virt

Provides fw_cfg for the virt machine on riscv.  This enables
using e.g.  ramfb later.

Signed-off-by: Asherah Connor <ashe@kivikakk.ee>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210318235041.17175-2-ashe@kivikakk.ee
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>