doc: clarify path selection in virtio guide
doc: clarify path selection in virtio guide

Add virtio paths selection and usage introduction for better
virtio usability.

Signed-off-by: Yinan Wang <49b1c4c1483798ccfe23a5285410aaacc037c97a@intel.com>
Acked-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