rate-limiter: add getRateLimiter/setRateLimiter in endpoint

We use tc-based or built-in rate limiter to shape network I/O traffic
and they all must be tied to one specific interface/endpoint.
In order to tell whether we've ever added rate limiter to this interface/endpoint,
we create get/set func to reveal/store such info.

Fixes: #250

Signed-off-by: Penny Zheng <penny.zheng@arm.com>

src/runtime/virtcontainers/endpoint.go

@@ -29,6 +29,11 @@ type Endpoint interface {
 
 	save() persistapi.NetworkEndpoint
 	load(persistapi.NetworkEndpoint)
+
+	GetRxRateLimiter() bool
+	SetRxRateLimiter() error
+	GetTxRateLimiter() bool
+	SetTxRateLimiter() error
 }
 
 // EndpointType identifies the type of the network endpoint.