KVM: x86: Clear output regs for CPUID 0x14 if PT isn't exposed to guest

Clear the output regs for the main CPUID 0x14 leaf (index=0) if Intel PT
isn't exposed to the guest.  Leaf 0x14 enumerates Intel PT capabilities
and should return zeroes if PT is not supported.  Incorrectly reporting
PT capabilities is essentially a cosmetic error, i.e. doesn't negatively
affect any known userspace/kernel, as the existence of PT itself is
correctly enumerated via CPUID 0x7.

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>