nestedsvm: Support TSC Rate MSR

nestedsvm: Support TSC Rate MSR

Support TSC Rate MSR and enable TSC scaling for
nested virtualization.

With it, guest VMs don't need take #VMEXIT to calculate a translated
TSC value when it is running under TSC emulation mode.

I measured native performance of the rdtsc instruction
in the l2 guest with xen-on-xen and both host and
and l1 guest run under TSC emulation mode.

TSC scaling just needs MSR emulation and correct tsc offset
calculation to be done and thus can be emulated also on older
hardware. In this case rdtsc instruction is intercepted and
handled by the host directly and safes the cost of a full
VMRUN/VMEXIT emulation cycle.

Signed-off-by: Christoph Egger <Christoph.Egger@amd.com>