KVM: SVM: fix trashing of MSR_TSC_AUX

I don't know what I was thinking when I wrote commit 46896c73 ("KVM:
svm: add support for RDTSCP", 2015-11-12); I missed write_rdtscp_aux which
obviously uses MSR_TSC_AUX.

Therefore we do need to save/restore MSR_TSC_AUX in svm_vcpu_run.

Cc: stable@vger.kernel.org
Cc: Borislav Petkov <bp@alien8.de>
Fixes: 46896c73

 ("KVM: svm: add support for RDTSCP")
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>