i386: Propagate SGX CPUID sub-leafs to KVM

The SGX sub-leafs are enumerated at CPUID 0x12.  Indices 0 and 1 are
always present when SGX is supported, and enumerate SGX features and
capabilities.  Indices >=2 are directly correlated with the platform's
EPC sections.  Because the number of EPC sections is dynamic and user
defined, the number of SGX sub-leafs is "NULL" terminated.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Yang Zhong <yang.zhong@intel.com>
Message-Id: <20210719112136.57018-15-yang.zhong@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>