Set the a vhdrlen > 0 for tap and report the VIRTIO_NET_F_CSUM feature if transmission checksum is enabled on the tap interface. This allows to use checksum offloading for packets to and from the VM.
To get the tap interface capabilities, the added code uses the ioctrl SIOCGIFCAP, which is added by D51289.
A user must enable txcsum on the tap interface before, which is possible with the code changes by D51291.
Once vhdrlen > 0, mbuf flags are translated in virtio network header flags and vice versa. D51686 fixes these translations.