Allow VF vlanid to be passed as a pointer

Allow VF vlanid to be passed as a pointer

There should be a way to show no intent in programming a VLAN at all
(including clearing it). This allows handling error conditions
differently when VLAN clearing is explicit (vlan id == 0) vs implicit
(vlanid == NULL - try to clear it if possible).

Signed-off-by: Dmitrii Shcherbakov <dmitrii.shcherbakov@canonical.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>