conf: Change return value of some CPU feature APIs

conf: Change return value of some CPU feature APIs

The virCPUDefAddFeatureInternal helper function only fails if it is
called with VIR_CPU_ADD_FEATURE_MODE_EXCLUSIVE, which is only used in
virCPUDefAddFeature. The other callers (virCPUDefUpdateFeature and
virCPUDefAddFeatureIfMissing) will never get anything but 0 from
virCPUDefAddFeatureInternal and their return type can be changed to
void.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>