riscv: qemu: enable distro boot from scsi

Booting via distro boot fails for:

    qemu-system-riscv64
    -drive if=none,file=sct-riscv64.img,format=raw,id=mydisk \
    -device ich9-ahci,id=ahci -device ide-hd,drive=mydisk,bus=ahci.0

Enable distro booting from an attached SCSI disk.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Bin Meng <bin.meng@windriver.com>