The function is supposed to add a feature to a CPU definition, let's
name it virCPUDefAddFeatureInternal. The behavior in case the feature is
already present in the CPU def is configurable and we will soon add a
new option to not do anything in that case, which wouldn't really work
well with the current *Update* name.
Signed-off-by: Jiri Denemark <jdenemar@redhat.com> Reviewed-by: Tim Wiederhake <twiederh@redhat.com>