Based on the LKML feedback, the devices under /dev/sgx/* are
not justified. SGX RFC v40 moves the SGX device nodes to /dev/sgx_*
and this is reflected in kvm-sgx (next branch) too.
Update cloud-hypervisor code and documentation to follow this.
Signed-off-by: Mikko Ylinen <mikko.ylinen@intel.com>