Skip virtual interfaces when parsing route table

Since routing through virtual interfaces may trigger DNS loop in VPN
like observing in UnifiOS Site Magic VPN.

nameservers_linux_test.go

@@ -0,0 +1,10 @@
+package ctrld
+
+import (
+	"testing"
+)
+
+func Test_virtualInterfaces(t *testing.T) {
+	vis := virtualInterfaces()
+	t.Log(vis)
+}