qemu-io qemu-nbd: Use error_report() etc. instead of fprintf()

Just three instances left.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1450452927-8346-16-git-send-email-armbru@redhat.com>
diff --git a/qemu-nbd.c b/qemu-nbd.c
index f9fce4a..99df01f 100644
--- a/qemu-nbd.c
+++ b/qemu-nbd.c
@@ -257,7 +257,7 @@
     fd = open(device, O_RDWR);
     if (fd < 0) {
         /* Linux-only, we can use %m in printf.  */
-        fprintf(stderr, "Failed to open %s: %m\n", device);
+        error_report("Failed to open %s: %m", device);
         goto out_socket;
     }