virtio: add vhost sw_if_index filter for sw_interface_vhost_user_dump

The filter sw_if_index was in the API sw_interface_vhost_user_dump. But
it was never implemented in the backend. This patch is to add the backend,
vat, and custom dump support for the filter.

Type: feature

Signed-off-by: Steven Luong <sluong@cisco.com>
Change-Id: Iaa41a7e11bfbcbb4c60092375e4b0dcf0950077b
diff --git a/src/vnet/devices/virtio/vhost_user.api b/src/vnet/devices/virtio/vhost_user.api
index 127b0a2..338fd71 100644
--- a/src/vnet/devices/virtio/vhost_user.api
+++ b/src/vnet/devices/virtio/vhost_user.api
@@ -115,7 +115,7 @@
 };
 
 /** \brief Vhost-user interface dump request
-    @param sw_if_index - filter by sw_if_index UNIMPLEMENTED
+    @param sw_if_index - filter by sw_if_index
 */
 define sw_interface_vhost_user_dump
 {