netlink: add support for multi-part netlink messages.

Such messages do not have NLMSG_ERROR or NLMSG_DONE type
but they are valid responses. We test 'multi-partness'
by looking for NLM_F_MULTI flag.

src/util/virnetlink.c

@@ -394,7 +394,9 @@ virNetlinkGetErrorCode(struct nlmsghdr *resp, unsigned int recvbuflen)
         break;
 
     default:
-        goto malformed_resp;
+        /* We allow multipart messages. */
+        if (!(resp->nlmsg_flags & NLM_F_MULTI))
+            goto malformed_resp;
     }
 
     return result;