virtio: add packet buffering on tx

Type: feature

This patch adds packet buffering on tx for
slow backend which have some jitter/delays
in freeing the vrings.

There are some limitations to the current design:
1) It only works in poll mode.
2) Atleast 1 rx queue of an interface (with buffering
   enabled) should be placed on each worker and main thread.

Signed-off-by: Mohsin Kazmi <sykazmi@cisco.com>
Change-Id: Ib93c350298b228e80426e58ac77f3bbc93b8be27
diff --git a/src/vnet/CMakeLists.txt b/src/vnet/CMakeLists.txt
index 4c65d71..0bc55f9 100644
--- a/src/vnet/CMakeLists.txt
+++ b/src/vnet/CMakeLists.txt
@@ -989,6 +989,7 @@
 list(APPEND VNET_HEADERS
   devices/virtio/pci.h
   devices/virtio/virtio.h
+  devices/virtio/virtio_buffering.h
   devices/virtio/virtio_std.h
   devices/virtio/virtio_pci_legacy.h
   devices/virtio/virtio_pci_modern.h