riscv: enable CONFIG_CMD_SBI for QEMU boards

Let CONFIG_TARGET_QEMU_VIRT imply CONFIG_CMD_SBI.

The sbi command provides detailed information about the SBI.
It is useful to test the discovery of extensions.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>