x86/cpuid: Recalculate a domains CPUID policy when appropriate

x86/cpuid: Recalculate a domains CPUID policy when appropriate

Introduce recalculate_cpuid_policy() which clamps a CPUID policy based on the
domains current restrictions.

Each adjustment introduced here mirrors what currently happens in
{pv,hvm}_cpuid(), although some logic is expressed differently.

 * The clearing X86_FEATURE_LM for 32bit PV guests, sanitise_featureset()
   takes out all 64bit-dependent features in one go.

 * The toolstacks choice of X86_FEATURE_ITSC in (by default) clobbered in
   domain_cpuid(), but {pv,hvm}_cpuid() needed to account for the host ITSC
   value when masking the toolstack value.

This now requires that sanitise_featureset(), lookup_deep_deps() and
associated data needs to be available at runtime, so moves out of __init.

Recalculate the cpuid policy when:

 * The domain is first created
 * Switching a PV guest to being compat
 * Setting disable_migrate or vTSC modes
 * The toolstack sets new policy data

The disable_migrate code was previously common.  To compensate, move the code
to each archs arch_do_domctl(), as the implementations now differ.

From this point on, domains have full and correct feature-leaf information in
their CPUID policies, allowing for substantial cleanup and improvements.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>