Implementing CoVE spec

[wojciechozga]

This PR brings changes to the security monitor required by the RISC-V CoVE spec. Specifically, security monitor supports now a subset of COVE SBI calls (COVH, COVG) required to implement CoVE deployment model 3. Security monitor also implements support for the NACL shared memory. With this PR, the hypervisor and test linux guest use modified Linux patches that introduce CoVE to the Linux kernel 1. Our changes to the Linux kernel are still work in progress.