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/vhostuser_endpoint.go

@@ -169,3 +169,21 @@ func (endpoint *VhostUserEndpoint) load(s persistapi.NetworkEndpoint) {
 		endpoint.PCIAddr = s.VhostUser.PCIAddr
 	}
 }
+
+// unsupported
+func (endpoint *VhostUserEndpoint) GetRxRateLimiter() bool {
+	return false
+}
+
+func (endpoint *VhostUserEndpoint) SetRxRateLimiter() error {
+	return fmt.Errorf("rx rate limiter is unsupported for vhost user endpoint")
+}
+
+// unsupported
+func (endpoint *VhostUserEndpoint) GetTxRateLimiter() bool {
+	return false
+}
+
+func (endpoint *VhostUserEndpoint) SetTxRateLimiter() error {
+	return fmt.Errorf("tx rate limiter is unsupported for vhost user endpoint")
+}