With the new default Q35 chipset support introduced in VHI 6.3, newly created virtual machines will not boot if they are created from images without the virtio-blk driver installed.
To prevent using the Q35 chipset in such cases, an image should have a special property requesting the legacy machine type 'pc'.
You can set this property for an image by using the following command:
openstack --insecure image set <image_id> --property hw_machine_type=pc