tools/cpuid: Untangle Invariant TSC handling

tools/cpuid: Untangle Invariant TSC handling

ITSC being visible to the guest is currently implicit with the toolstack
unconditionally asking for it, and Xen clipping it based on the vTSC and/or
XEN_DOMCTL_disable_migrate settings.

This is problematic for several reasons.

First, the implicit vTSC behaviour manifests as a real bug on migration to a
host with a different frequency, with ITSC but without TSC scaling
capabilities, whereby the ITSC feature becomes advertised to the guest.  ITSC
will disappear again if the guest migrates to server with the same frequency
as the original, or to one with TSC scaling support.

Secondly, disallowing ITSC unless the guest doesn't migrate is conceptually
wrong.  It is common to have migration pools of identical hardware, at which
point the TSC frequency is nominally the same, and more modern hardware has
TSC scaling support anyway.  In both cases, it is safe to advertise ITSC and
migrate the guest.

Remove all implicit logic in Xen, and make ITSC part of the max CPUID policies
for guests.  Plumb an itsc parameter into xc_cpuid_apply_policy() and have
libxl__cpuid_legacy() fill in the two cases where it can reasonably expect
ITSC to be safe for the guest to see.  This retains the current side effect of
enabling ITSC if the guest is marked as nomigrate.

This is a behaviour change for TSC_MODE_NATIVE, where the ITSC will now
reliably not appear, and for the case where the user explicitly requests ITSC,
in which case it will appear even if the guest isn't marked as nomigrate.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Jackson <Ian.Jackson@citrix.com>