Cross-pool migration of HVM Centos 7 fails from XS 6.5 to 7.6

This is a follow-up to XSO-924

I've made new tests with a VM that was created using the Cent0S 7 template:

• VM created from CentOS 7 template in XS 7.5 migrated fine to XS 7.6
• VM created from CentOS 7 template in XS 6.5 fails to migrate to XS 7.6: an error occurs and the VM gets duplicated in both servers (both claim that they have a VM with that uuid).

I uploaded status reports captured shortly after the failure for both XS 6.5 and XS 7.6:

• XS 6.5 (sender): 18bc0e14-19b4-11e9-a395-060d5b20952f
• XS 7.6 (receiver): 2db9e156-19b4-11e9-9960-062bccfcbe8f

This is cross-pool migration.