balloon: improve error msg when adding second device

A VM supports only one balloon device, but due to several changes
in infrastructure the error message got messed up when trying
to add a second device. Fix it.

Before this fix

Command-line:

qemu-qmp: -device virtio-balloon-pci,id=balloon0: Another balloon device already registered
qemu-qmp: -device virtio-balloon-pci,id=balloon0: Adding balloon handler failed
qemu-qmp: -device virtio-balloon-pci,id=balloon0: Device 'virtio-balloon-pci' could not be initialized

HMP:

Another balloon device already registered
Adding balloon handler failed
Device 'virtio-balloon-pci' could not be initialized

QMP:

{ "execute": "device_add", "arguments": { "driver": "virtio-balloon-pci", "id": "balloon0" } }
{
	"error": {
		"class": "GenericError",
		"desc": "Adding balloon handler failed"
	}
}

After this fix

Command-line:

qemu-qmp: -device virtio-balloon-pci,id=balloon0: Only one balloon device is supported
qemu-qmp: -device virtio-balloon-pci,id=balloon0: Device 'virtio-balloon-pci' could not be initialized

HMP:

(qemu) device_add virtio-balloon-pci,id=balloon0
Only one balloon device is supported
Device 'virtio-balloon-pci' could not be initialized
(qemu)

QMP:

{ "execute": "device_add",
          "arguments": { "driver": "virtio-balloon-pci", "id": "balloon0" } }
{
    "error": {
        "class": "GenericError",
        "desc": "Only one balloon device is supported"
    }
}
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>

balloon.c

@@ -58,7 +58,6 @@ int qemu_add_balloon_handler(QEMUBalloonEvent *event_func,
         /* We're already registered one balloon handler.  How many can
          * a guest really have?
          */
-        error_report("Another balloon device already registered");
         return -1;
     }
     balloon_event_fn = event_func;

hw/virtio/virtio-balloon.c

@@ -383,7 +383,7 @@ static void virtio_balloon_device_realize(DeviceState *dev, Error **errp)
                                    virtio_balloon_stat, s);
 
     if (ret < 0) {
-        error_setg(errp, "Adding balloon handler failed");
+        error_setg(errp, "Only one balloon device is supported");
         virtio_cleanup(vdev);
         return;
     }