virtio net attempts to peek into virtio queue to
determine that we have enough space for the complete
packet to fit. However, it fails to account for space
consumed by virtio net header when it does this,
under stress this results in a failure
with the message 'truncating packet'.

redhat bz 591494.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>