hw/net/net_rx_pkt: Remove net_rx_pkt_has_virt_hdr

hw/net/net_rx_pkt: Remove net_rx_pkt_has_virt_hdr

When virtio-net header is not set, net_rx_pkt_get_vhdr() returns
zero-filled virtio_net_hdr, which is actually valid. In fact, tap device
uses zero-filled virtio_net_hdr when virtio-net header is not provided
by the peer. Therefore, we can just remove net_rx_pkt_has_virt_hdr() and
always assume NetTxPkt has a valid virtio-net header.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>