KVM: svm: add support for RDTSCP

RDTSCP was never supported for AMD CPUs, which nobody noticed because
Linux does not use it.  But exactly the fact that Linux does not
use it makes the implementation very simple; we can freely trash
MSR_TSC_AUX while running the guest.

Cc: Joerg Roedel <joro@8bytes.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>