Properly support virtio on 64-bit RISC-V

[mkroening]

          I'm getting errors with the virtio devices when compiling the hermit-rs-template for the risc-v hermit target.

Originally posted by @marcomaas1990 in https://github.com/hermit-os/loader/issues/345#issuecomment-2122481598

[mkroening]

This should probably be virtio-mmio: https://www.qemu.org/docs/master/system/riscv/virt.html