hw/i386/fw_cfg: Set SGX bits in feature control fw_cfg accordingly

Request SGX an SGX Launch Control to be enabled in FEATURE_CONTROL
when the features are exposed to the guest. Our design is the SGX
Launch Control bit will be unconditionally set in FEATURE_CONTROL,
which is unlike host bios.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Yang Zhong <yang.zhong@intel.com>
Message-Id: <20210719112136.57018-17-yang.zhong@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>