Add new CONFIG_SGX for sgx support in the Qemu, and the Kconfig
default enable sgx in the i386 platform.

Signed-off-by: Yang Zhong <yang.zhong@intel.com>
Message-Id: <20210719112136.57018-32-yang.zhong@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>