Adds an order_by to the interfaces index

Adds an order_by on the created_at timestamp on a list of
InstanceInterfaces in an attempt to mitigate bug 1983, where guests
sometimes receive an out-of-order VIF list after a resize.

Change-Id: I3a52811a8afdee84e1263062247806bbf621a3f7
Merge pull request #42 from Cerberus98/vif_order_fix

Adds an order_by to the interfaces index