virtio-net: correct packet length math

virtio-net: correct packet length math

We were requesting too much when checking buffer
length: size already includes host header length.

Further, we should not exit if we get a packet that
is too long, since this might not be under control
of the guest. Just drop the packet.

Red Hat bz 591494

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>