virtio: Net header needs hdr_len

It's far easier to deal with packets if we don't have to parse the
packet to figure out the header length to know how much to pull into
the skb data.  Add the field to the virtio_net_hdr struct (and fix the
spaces that somehow crept in there).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>