Fix to allow compute driver attributes

Add the following compute driver attributes
- default_ephemeral_format
- preallocate_images
- use_cow_images
- vif_plugging_is_fatal
- vif_plugging_timeout

Change-Id: Icd014c1c88ab2d4c6773a4cbe14b638c2332fd4a
Closes-Bug: #1317564