KVM: VMX: Set up interception for CET MSRs

KVM: VMX: Set up interception for CET MSRs

Disable interception for CET MSRs that can be accessed via XSAVES/XRSTORS,
and exist accordingly to CPUID, as accesses through XSTATE aren't subject
to MSR interception checks, i.e. can't be intercepted without intercepting
and emulating XSAVES/XRSTORS, and KVM doesn't support emulating
XSAVE/XRSTOR instructions.

Don't condition interception on the guest actually having XSAVES as there
is no benefit to intercepting the accesses (when the MSRs exist).  The
MSRs in question are either context switched by the CPU on VM-Enter/VM-Exit
or by KVM via XSAVES/XRSTORS (KVM requires XSAVES to virtualization SHSTK),
i.e. KVM is going to load guest values into hardware irrespective of guest
XSAVES support.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
Tested-by: Mathias Krause <minipli@grsecurity.net>
Tested-by: John Allen <john.allen@amd.com>
Tested-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Xin Li (Intel) <xin@zytor.com>
Link: https://lore.kernel.org/r/20250919223258.1604852-17-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>