io: fix qio_channel_socket_accept err handling

When accept failed, we should setup errp with the reason. More
importantly, the caller may assume errp be non-NULL when error happens,
and not setting the errp may crash QEMU.

At the same time, move the trace_qio_channel_socket_accept_fail() after
the if check on EINTR. Two reasons:

1. when EINTR happened, it's not really a fault (we should just try
   again), so we should not log with an "accept failure".

2. trace_*() functions may overwrite errno, then the old errno will be
   missing. We need to either check errno before trace_*() calls, or
   reserve the errno.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <1501666880-10159-3-git-send-email-peterx@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

io/channel-socket.c

@@ -340,10 +340,11 @@ qio_channel_socket_accept(QIOChannelSocket *ioc,
     cioc->fd = qemu_accept(ioc->fd, (struct sockaddr *)&cioc->remoteAddr,
                            &cioc->remoteAddrLen);
     if (cioc->fd < 0) {
-        trace_qio_channel_socket_accept_fail(ioc);
         if (errno == EINTR) {
             goto retry;
         }
+        error_setg_errno(errp, errno, "Unable to accept connection");
+        trace_qio_channel_socket_accept_fail(ioc);
         goto error;
     }