RISC-V support

[stlankes]

@simonschoening wrote a prototype for RISC-V. We should integrate this prototype to our version of uhyve.

[simonschoening]

I am going to rebase the prototype and create a (draft) pull request (#338).

[marcomaas1990]

Is it available yet?

[mkroening]

Not yet. There's a draft PR available, so it was working at some point, but we are currently lacking hardware that has KVM support to continue that PR.