i386: Update SGX CPUID info according to hardware/KVM/user input

Expose SGX to the guest if and only if KVM is enabled and supports
virtualization of SGX.  While the majority of ENCLS can be emulated to
some degree, because SGX uses a hardware-based root of trust, the
attestation aspects of SGX cannot be emulated in software, i.e.
ultimately emulation will fail as software cannot generate a valid
quote/report.  The complexity of partially emulating SGX in Qemu far
outweighs the value added, e.g. an SGX specific simulator for userspace
applications can emulate SGX for development and testing purposes.

Note, access to the PROVISIONKEY is not yet advertised to the guest as
KVM blocks access to the PROVISIONKEY by default and requires userspace
to provide additional credentials (via ioctl()) to expose PROVISIONKEY.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Yang Zhong <yang.zhong@intel.com>
Message-Id: <20210719112136.57018-13-yang.zhong@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>