x86/xen/time: prefer tsc as clocksource when it is invariant

Kvm elects to use tsc instead of kvm-clock when it can detect that the
TSC is invariant.

(As of commit 7539b174

 ("x86: kvmguest: use TSC clocksource if
invariant TSC is exposed")).

Notable cloud vendors[1] and performance engineers[2] recommend that Xen
users preferentially select tsc over xen-clocksource due the performance
penalty incurred by the latter.  These articles are persuasive and
tailored to specific use cases.  In order to understand the tradeoffs
around this choice more fully, this author had to reference the
documented[3] complexities around the Xen configuration, as well as the
kernel's clocksource selection algorithm.  Many users may not attempt
this to correctly configure the right clock source in their guest.

The approach taken in the kvm-clock module spares users this confusion,
where possible.

Both the Intel SDM[4] and the Xen tsc documentation explain that marking
a tsc as invariant means that it should be considered stable by the OS
and is elibile to be used as a wall clock source.

In order to obtain better out-of-the-box performance, and reduce the
need for user tuning, follow kvm's approach and decrease the xen clock
rating so that tsc is preferable, if it is invariant, stable, and the
tsc will never be emulated.

[1] https://aws.amazon.com/premiumsupport/knowledge-center/manage-ec2-linux-clock-source/
[2] https://www.brendangregg.com/blog/2021-09-26/the-speed-of-time.html
[3] https://xenbits.xen.org/docs/unstable/man/xen-tscmode.7.html
[4] Intel 64 and IA-32 Architectures Sofware Developer's Manual Volume
    3b: System Programming Guide, Part 2, Section 17.17.1, Invariant TSC

Signed-off-by: Krister Johansen <kjlx@templeofstupid.com>
Code-reviewed-by: David Reaver <me@davidreaver.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20221216162118.GB2633@templeofstupid.com
Signed-off-by: Juergen Gross <jgross@suse.com>