Although it wouldn't help making a consistent snapshot of an instance with multiple disks, for a single disk it shouldn't matter if the guest is quiesced as long as the backend can make an instantaneous snapshot. I'm pretty sure many (most?) backends would support that; certainly plain old LVM does. Does cinder use this functionality where it's available, and would that solve the problem you're trying to address?
Matt
That is a good point. If you snap all of the volumes used, it may not be quiesced and have all IO flushed, but it would at least be a crash consistent set of data. All of the volumes used by a VM would need to be added to a group: https://docs.openstack.org/cinder/latest/admin/blockstorage-groups.html You would then be able to create a group snapshot to have all volumes snapped together.