virnetlink: Split virNetlinkBridgeVlanFilterSet()

virnetlink: Split virNetlinkBridgeVlanFilterSet()

Currently, virNetlinkBridgeVlanFilterSet() takes @cmd as the
second argument where either RTM_SETLINK or RTM_DELLINK is
expected. Both of these constants come from linux/rtnetlink.h and
thus are undefined when building without netlink. This design
also clashes with the whole point of virnetlink: to offload
netlink dependency onto a single file.

Therefore, drop the argument, turn
virNetlinkBridgeVlanFilterSet() into just setter, effectively,
and introduce virNetlinkBridgeVlanFilterDel() for the case when
RTM_DELLINK would be passed as @cmd.

Resolves: https://gitlab.com/libvirt/libvirt/-/issues/770
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>