kvm: x86: Host feature SSBD doesn't imply guest feature AMD_SSBD

The host reports support for the synthetic feature X86_FEATURE_SSBD
when any of the three following hardware features are set:
  CPUID.(EAX=7,ECX=0):EDX.SSBD[bit 31]
  CPUID.80000008H:EBX.AMD_SSBD[bit 24]
  CPUID.80000008H:EBX.VIRT_SSBD[bit 25]

Either of the first two hardware features implies the existence of the
IA32_SPEC_CTRL MSR, but CPUID.80000008H:EBX.VIRT_SSBD[bit 25] does
not. Therefore, CPUID.80000008H:EBX.AMD_SSBD[bit 24] should only be
set in the guest if CPUID.(EAX=7,ECX=0):EDX.SSBD[bit 31] or
CPUID.80000008H:EBX.AMD_SSBD[bit 24] is set on the host.

Fixes: 4c6903a0

 ("KVM: x86: fix reporting of AMD speculation bug CPUID leaf")
Signed-off-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Jacob Xu <jacobhxu@google.com>
Reviewed-by: Peter Shier <pshier@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: stable@vger.kernel.org
Reported-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>