Hypervisor extension

[defermelowie]

This is a version of the hypervisor extension ported on a more recent version of the model. Feel welcome to provide any feedback

In order to verify the modifications & additions to this version of the model, the following tests are used:

• The compiled tests in ./test/riscv-tests/ in order to check for regressions
• By booting Linux on top of OpenSBI (w/o virtualization) in order to check for regressions
• A collection of test written by me for the H-extension: https://github.com/defermelowie/riscv-hext-asm-tests
• A fork of a testsuite written by José Martins for the H-extension: https://github.com/josecm/riscv-hyp-tests
• A fork of https://github.com/dramforever/riscv-hs-tests
• By booting Linux on top of Xvisor on top of OpenSBI (My current setup requires terminal input which is not already implemented nor part of the hypervisor spec.)

PS I closed #334 in favor of this pull request.