RISC-V Hypervisor extension support

[edubart]

Context

The RISC-V Hypervisor extension will allows us to run multiple virtual Linux guest machines inside the same Cartesi Machine, this open possibilities of running multiple process each one in a fully isolated Linux kernel, greatly improving security for dapps that needs to run multiple isolated processes.

Possible solutions

The latest ratified RISC-V Hypervisor extension spec should be read and implemented accordingly. We should also update to Kernel 6.x to have KVM support for RISC-V and switch our boot to use OpenSBI instead of BBL.