hw/riscv: allow ramfb on virt

hw/riscv: allow ramfb on virt

Allow ramfb on virt.  This lets `-device ramfb' work.

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: 20210318235041.17175-3-ashe@kivikakk.ee
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>