Rust vmm support for s390x

Rust vmm is around for quite a while, but upstream still largely supported for amd64 and arm64 only.
Even if several components compile on other architectures than these (incl. s390x),
it seems to be more a gift from Rust rather than a real indication that it works on this particular architecture.

Since there is increased popularity in Rust vmm, it would be great to see the s390x architecture supported by Rust vmm as well, which probably requires involvement of the architecture owner.