virtio_balloon: fix PFN format for virtio-1

Everything should be LE when using virtio-1, but
the linux balloon driver does not seem to care about that.

Reported-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>