Could you try to reproduce the error with virsh commands?
1. Create a VM as in your original reproduction.
2. Instead of creating a snapshot from Horizon. Go the compute where the instance is running and try to quiesce the domain with virsh:
virsh domfsfreeze <name of the domain>
3. If #2. is successful then try unquiesce it with
virsh domfsthaw <name of the domain>
4. If both #2. and #3. is successful then try to create a snapshot the same way as you did in your original reproduction.
Could you try to reproduce the error with virsh commands?
1. Create a VM as in your original reproduction.
2. Instead of creating a snapshot from Horizon. Go the compute where the instance is running and try to quiesce the domain with virsh:
virsh domfsfreeze <name of the domain>
3. If #2. is successful then try unquiesce it with
virsh domfsthaw <name of the domain>
4. If both #2. and #3. is successful then try to create a snapshot the same way as you did in your original reproduction.