drivers/net/virtio_net: Fixed padded vheader to use v1 with hash.

The header v1 provides additional info about RSS.
Added changes to computing proper header length.
In the next patches, the header may contain RSS hash info
for the hash population.

Signed-off-by: Andrew Melnychenko <andrew@daynix.com>
Link: https://lore.kernel.org/r/20220328175336.10802-2-andrew@daynix.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>