x86ModelParseFeatures: Don't construct list using 'virStringListAdd'

x86ModelParseFeatures: Don't construct list using 'virStringListAdd'

Pre-allocate the list to the upper bound and fill it gradually. Since
the data is kept long-term and the list won't be populated much shrink
it to the actual size after parsing.

While using 'virStringListAdd' here wouldn't be as expensive as this
function is used just once, the removal will allow to remove
'virStringListAdd' altogether to discourage the antipattern it promotes.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>