virtio net: Allow receiving SG packets

Finally this patch lets virtio_net receive GSO packets in addition
to sending them.  This can definitely be optimised for the non-GSO
case.  For comparison the Xen approach stores one page in each skb
and uses subsequent skb's pages to construct an SG skb instead of
preallocating the maximum amount of pages per skb.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> (added feature bits)