KVM: x86: Use KVM cpu caps to detect MSR_TSC_AUX virt support

Check for MSR_TSC_AUX virtualization via kvm_cpu_cap_has() and drop
->rdtscp_supported().

Note, vmx_rdtscp_supported() needs to hang around a tiny bit longer due
other usage in VMX code.

No functional change intended.

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>