This might cause a problem if you update an oVirt host that's been provisioned via images - could you try?

My worry is that `host.provision_method` isn't persisted (it's just an attr_accessor) so on updating an oVirt host that's provisioned via images, it will trigger this validator as provision_method will be `nil`.

Would `... and (!capabilities.include?(:image) || host.provision_method == 'build')` make more sense?