riscv: qemu: enable booting on a second virtio device

riscv: qemu: enable booting on a second virtio device

QEMU RISC-V supports multiple virtio devices, but only tries to boot to
the first one. Enable support for a second virtio device, that is useful
for instance to boot on a disk image + an installer. Ideally that should
be made dynamic, but that's a first step.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>