XenAPI:  Increase DomU's memory

Devstack has been increasing in complexity, and if we use too
much of our memory it can lead to fragmentation which in turn
causes a DomU error and a failure of random tests.

Change-Id: Ide9cc84625caed4e35a64a47ee1e92a6cd567651
Merge "XenAPI:  Increase DomU's memory"