net: add tap_has_vnet_hdr() and tap_using_vnet_hdr() APIs These lamely named functions allow virtio-net to query whether IFF_VNET_HDR is enabled on a tap interface and inform the tap code that virtio-net will supply packets with a vnet header. Signed-off-by: Mark McLoughlin <markmc@redhat.com> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>