x86/cpuid: Factor common parsing out of parse_xen_cpuid()

x86/cpuid: Factor common parsing out of parse_xen_cpuid()

dom0-cpuid= is going to want to reuse the common parsing loop, so factor it
out into parse_cpuid().

Irritatingly, despite being static const, the features[] array gets duplicated
each time parse_cpuid() is inlined.  As it is a large (and ever growing with
new CPU features) datastructure, move it to being file scope so all inlines
use the same single object.

No functional change.

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