KVM: SVM: Disable interception of SPEC_CTRL iff the MSR exists for the guest

KVM: SVM: Disable interception of SPEC_CTRL iff the MSR exists for the guest

Disable interception of SPEC_CTRL when the CPU virtualizes (i.e. context
switches) SPEC_CTRL if and only if the MSR exists according to the vCPU's
CPUID model.  Letting the guest access SPEC_CTRL is generally benign, but
the guest would see inconsistent behavior if KVM happened to emulate an
access to the MSR.

Fixes: d00b99c514b3 ("KVM: SVM: Add support for Virtual SPEC_CTRL")
Reported-by: Chao Gao <chao.gao@intel.com>
Link: https://lore.kernel.org/r/20250610225737.156318-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>