Fix virtio/net_device receive to support iovec.offset>0

Because virtio does not provide a way to specify offsets we use
memmove(3) to adjust the iovec to have offset=0 before sending.

Fixes the problem reported on this mailing list thread:
  https://groups.google.com/forum/#!topic/snabb-devel/ar778eECIL4
Merge pull request #281 from lukego/l2tpv3-virtio-fix

Fix virtio/net_device receive to support iovec.offset>0