proxy: decouple from sandbox

A proxy is mostly associated with an agent. Decouple it from sandbox
so that we can start it before linking vm with an actual sandbox.

Signed-off-by: Peng Tao <bergwolf@gmail.com>

virtcontainers/cc_proxy_test.go

@@ -12,5 +12,5 @@ import (
 func TestCCProxyStart(t *testing.T) {
 	proxy := &ccProxy{}
 
-	testProxyStart(t, nil, proxy, CCProxyType)
+	testProxyStart(t, nil, proxy)
 }