riscv: qemu: Enumerate virtio bus during early boot

Currently devices on the virtio bus is not automatically enumerated,
which means peripherals on the virtio bus are not discovered by their
drivers. This uses board_init() to do the virtio enumeration.

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