tap: vhost busy polling support

This patch add the capability of basic vhost net busy polling which is
supported by recent kernel. User could configure the maximum number of
us that could be spent on busy polling through a new property of tap
"poll-us".

Cc: Greg Kurz <groug@kaod.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

qapi-schema.json

@@ -2592,6 +2592,9 @@
 #
 # @queues: #optional number of queues to be created for multiqueue capable tap
 #
+# @poll-us: #optional maximum number of microseconds that could
+# be spent on busy polling for tap (since 2.7)
+#
 # Since 1.2
 ##
 { 'struct': 'NetdevTapOptions',
@@ -2608,7 +2611,8 @@
     '*vhostfd':    'str',
     '*vhostfds':   'str',
     '*vhostforce': 'bool',
-    '*queues':     'uint32'} }
+    '*queues':     'uint32',
+    '*poll-us':    'uint32'} }
 
 ##
 # @NetdevSocketOptions