Description of problem:

When hot plugging a disk the cache mode is not set explicitly which can result in a default cache mode being used. We would like to use the same logic for hot plugged disks as for conventionally attached disks which is to prefer cache=none when the underlying storage supports direct IO.

Version-Release number of selected component (if applicable): 4.10.7

How reproducible: Always

Steps to Reproduce:
1. Create VM
2. Start VM
3. Hotplug a disk
4. Use `oc rsh` to connect to the appropriate virt-launcher Pod
5. Execute `virsh dumpxml 1`

Actual results: The xml shows a cache mode other than 'none' for the hotplug disk

Expected results: The cache mode should be set to 'none'

Additional info: