kvmi-v6: unable to set special registers and MSRs

[Wenzel]

While implementing VMI_EVENT_RESPONSE_SET_REGISTERS in Libvmi KVM driver, I realized that it wasn't possible to set special registers and MSRs.

The API provided by kvmi-v6 allows to get these registers: https://github.com/KVM-VMI/kvm/blob/5205f803008a2ee5788ba0a9cc7a475a546889ba/tools/kvm/kvmi/include/kvmi/libkvmi.h#L110

but not to set them: https://github.com/KVM-VMI/kvm/blob/5205f803008a2ee5788ba0a9cc7a475a546889ba/tools/kvm/kvmi/include/kvmi/libkvmi.h#L112