virtio_mmio: support non-legacy balloon devices

virtio_device_is_legacy_only is always false now,
drop the test from virtio mmio.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Pawel Moll <pawel.moll@arm.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>