virtio-net: Do not filter VLANs without F_CTRL_VLAN

virtio-net: Do not filter VLANs without F_CTRL_VLAN

If VIRTIO_NET_F_CTRL_VLAN is not negotiated, do not filter out all
VLAN-tagged packets but send them to the guest.

This fixes VLANs with OpenBSD guests (and probably NetBSD, too, because
the OpenBSD driver started as a port from NetBSD).

Signed-off-by: Stefan Fritsch <sf@sfritsch.de>
Signed-off-by: Amos Kong <akong@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>