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: Luiz Capitulino <lcapitulino@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
