x86: Avoid corruption on migrate for vcpus using CPUID Faulting

x86: Avoid corruption on migrate for vcpus using CPUID Faulting

Xen 4.8 and later virtualises CPUID Faulting support for guests.  However, the
value of MSR_MISC_FEATURES_ENABLES is omitted from the vcpu state, meaning
that the current cpuid faulting setting is lost on migrate/suspend/resume.

Instead of following the MSR status quo, take the opportunity to make the
logic more generic, and in particular, trivial to extend for future MSRs.

This is done by discarding the notion of optional MSRs, and requiring the
toolstack to be prepared to move all of the MSRs, although only a subset will
typically need to move.

This allows for the use of guest_{rd,wr}msr() alone to evaluate whether an MSR
needs moving.  This is a benefit because it means there is a single piece of
logic responsible for evaluating whether a guest can use an MSR, and which
values are acceptable.

One small adjustment to guest_wrmsr() is required to cope with being called in
toolstack context.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Julien Grall <julien.grall@linaro.org>