runtime: add support for SGX

Support the `sgx.intel.com/epc` annotation that is defined by the intel
k8s plugin. This annotation enables SGX. Hardware-based isolation and
memory encryption.

For example, use `sgx.intel.com/epc = "64Mi"` to create a container
with 1 EPC section with pre-allocated memory.

At the time of writing this patch, SGX patches have not landed on the
linux kernel project.
The following github kernel fork contains all the SGX patches for the
host and guest: https://github.com/intel/kvm-sgx

fixes #483

Signed-off-by: Julio Montes <julio.montes@intel.com>

src/runtime/virtcontainers/persist/api/config.go

@@ -185,6 +185,10 @@ type HypervisorConfig struct {
 
 	// TxRateLimiterMaxRate is used to control network I/O outbound bandwidth on VM level.
 	TxRateLimiterMaxRate uint64
+
+	// SGXEPCSize specifies the size in bytes for the EPC Section.
+	// Enable SGX. Hardware-based isolation and memory encryption.
+	SGXEPCSize int64
 }
 
 // KataAgentConfig is a structure storing information needed