channel-posix: using qemu_set_nonblock() instead of fcntl(O_NONBLOCK)

Technically, fcntl(soc, F_SETFL, O_NONBLOCK)
is incorrect since it clobbers all other file flags.
We can use F_GETFL to get the current flags, set or
clear the O_NONBLOCK flag, then use F_SETFL to set the flags.

Using the qemu_set_nonblock() wrapper.

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Signed-off-by: Wangxin <wangxinxin.wang@huawei.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

qga/channel-posix.c

@@ -42,7 +42,7 @@ static gboolean ga_channel_listen_accept(GIOChannel *channel,
         g_warning("error converting fd to gsocket: %s", strerror(errno));
         goto out;
     }
-    fcntl(client_fd, F_SETFL, O_NONBLOCK);
+    qemu_set_nonblock(client_fd);
     ret = ga_channel_client_add(c, client_fd);
     if (ret) {
         g_warning("error setting up connection");