x86/time: don't use virtual TSC if host and guest frequencies are equal

x86/time: don't use virtual TSC if host and guest frequencies are equal

Commit 82713ec8d2 ("x86: use native RDTSC(P) execution when guest and
host frequencies are the same") left out optimization for PV guests
when host and guest run at the same frequency.

For such a case we should be able not to use virtual TSC regardless
of whether we are runing before or after a migration (i.e. regardless
of incarnation value).

Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
[jb: retain parts of the original comment]
Reviewed-by: Jan Beulich <jbeulich@suse.com>