virtio: fix scatterlist sizing in net driver.

Herbert Xu points out (within another patch) that my scatterlists are
too short: one entry for the gso header, one for the skb->data, and
MAX_SKB_FRAGS for all the fragments.

Fix both xmit and recv sides (recv currently unused, coming in later
patch).

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