Add riscv64 support

[kxxt]

Summary of the PR

Add riscv64 support.

Requirements

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  summary and max 75 characters for each description line.
• All added/changed functionality has a corresponding unit/integration
  test.
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• Any newly added unsafe code is properly documented.

[alindima]

thanks for the contribution. while it overall looks good, we don't have any riscv support in the CI. Without that, I don't think we can merge such PRs, since it would basically result in untested code being released.
I noticed at least one other rust-vmm crate which has open PRs for riscv support also. Is there such planned support?
CC: @andreeaflorescu (mentioning you since you may have an opinion on this)

[kxxt]

while it overall looks good,

Thank!

while it overall looks good, we don't have any riscv support in the CI.

It looks like there's a PR adding that: rust-vmm/rust-vmm-ci#148

Without that, I don't think we can merge such PRs, since it would basically result in untested code being released.

I think another possible way could be merging riscv64 support and labeling riscv64 as a community supported and not officially tested architecture in the documentation.

BTW here is a test log on riscv64: https://paste.rs/Oyi0p