hw/virtio/virtio-balloon: move adding property to virtio_balloon_instance_init

hw/virtio/virtio-balloon: move adding property to virtio_balloon_instance_init

This is in preparation for using alias property in virtio-balloon-pci
and virtio-balloon-ccw.

Signed-off-by: Shannon Zhao <zhaoshenglong@huawei.com>
Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>