mgcp_protocol: Avoid code duplication between virtual + other trunks

There were two code paths that were supposed to do exactly the same,
but then in Change-Id I3994af016fb96427263edbba05f560743f85fdd4 only
one of the two was modified, resulting in OS#4034

Let's
* dynamically allocate the virtual trunk
* rename mgcp_config.trunk to mgcp_config.virt_trunk to clarify
* as a result, abolish copy+pasted code for trunk initialization

Change-Id: I54762af6d417b849a24b6e71b6c5c996a5cb3fa6
Related: OS#4034
diff --git a/src/libosmo-mgcp/mgcp_osmux.c b/src/libosmo-mgcp/mgcp_osmux.c
index a1121de..8da7361 100644
--- a/src/libosmo-mgcp/mgcp_osmux.c
+++ b/src/libosmo-mgcp/mgcp_osmux.c
@@ -202,9 +202,9 @@
 	struct mgcp_conn_rtp * conn_rtp;
 	int i;
 
-	for (i=0; i<cfg->trunk.number_endpoints; i++) {
+	for (i=0; i<cfg->virt_trunk->number_endpoints; i++) {
 
-		endp = &cfg->trunk.endpoints[i];
+		endp = &cfg->virt_trunk->endpoints[i];
 
 		llist_for_each_entry(conn, &endp->conns, entry) {
 			if (conn->type != MGCP_CONN_TYPE_RTP)