x86: use native RDTSC(P) execution when guest and host frequencies are the same

x86: use native RDTSC(P) execution when guest and host frequencies are the same

We should be able to continue using native RDTSC(P) execution on
HVM/PVH guests after migration if host and guest frequencies are
equal (this includes the case when the frequencies are made equal
by TSC scaling feature).

This also allows us to revert main part of commit 4aab59a3 (svm: Do not
intercept RDTSC(P) when TSC scaling is supported by hardware) which
was wrong: while RDTSC intercepts were disabled domain's vtsc could
still be set, leading to inconsistent view of guest's TSC.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Acked-by: Jan Beulich <jbeulich@suse.com>