qemu: Get SGX capabilities form QMP

qemu: Get SGX capabilities form QMP

Generate the QMP command for query-sgx-capabilities and the command
return SGX capabilities from QMP.

{"execute":"query-sgx-capabilities"}

the right reply:
  {"return":
    {
      "sgx": true,
      "section-size": 197132288,
      "flc": true
    }
  }

the error reply:
  {"error":
    {"class": "GenericError", "desc": "SGX is not enabled in KVM"}
  }

Signed-off-by: Haibin Huang <haibin.huang@intel.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>