vm/qemu: add support for linux/riscv64
vm/qemu: add support for linux/riscv64

Since the qemu riscv64 port does not work with the default -net nic
parameter, add support to use in the modern way of specifying network devices
to qemu, i.e. -device virtio-net-device,netdev=0 -netdev user,id=net0,host=...

The same applies for the -hda qemu option. Provide a way to use the
modern way of specifying image devices as well.

Other/new ports may use these options as well in the future by setting
UseNewQemuNetOptions and/or UseNewQemuImageOptions.

Signed-off-by: Tobias Klauser <9249c549618448e4c699e8032ba6c7f0d7fd4b7f@distanz.ch>

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