net/virtio: optimize Tx enqueue for packed ring
net/virtio: optimize Tx enqueue for packed ring

This patch introduces an optimized enqueue function in packed
ring for the case that virtio net header can be prepended to
the unchained mbuf.

Signed-off-by: Tiwei Bie <f338cf29d7d69ff1bc99f755ebfb0eb0accd409e@intel.com>
Reviewed-by: Maxime Coquelin <9a2667dee3b90866bbc9c2696cff2b7d5616c95a@redhat.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk