libx86: Helper for clearing out-of-range CPUID leaves

libx86: Helper for clearing out-of-range CPUID leaves

When merging a levelled policy, stale out-of-range leaves may remain.
Introduce a helper to clear them, and test a number of the subtle corner
cases.

The logic based on cpuid_policy_xstates() is liable to need changing when XCR0
has bit 63 defined.  Leave BUILD_BUG_ON()'s behind with comments in all all
impacted areas, which includes in x86_cpuid_policy_fill_native().

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