x86/cpuid: Introduce dom0-cpuid command line option

x86/cpuid: Introduce dom0-cpuid command line option

Specifically, this lets the user opt in to non-default features.

Collect all dom0 settings together in dom0_{en,dis}able_feat[], and apply it
to dom0's policy when other tweaks are being made.

As recalculate_cpuid_policy() is an expensive action, and dom0-cpuid= is
likely to only be used by the x86 maintainers for development purposes, forgo
the recalculation in the general case.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit 5bd2b82df28cb7390f5ffb00fac635d0b9e36674)