Current check_can_live_migrate_destination trys to get instance info
by call self._conn.lookupByName(instance_name) on the destination
host before migrating. Because the instance is not on the destination
host yet, it failes with InstanceNotFound.
This fix gets the available disk size of the destination side.
On the source side, checks whether the above space is enough before
migrating.
Fixes bug 1044237
Change-Id: I20f64e1f85828049b697a4b1173bac8e5779d45a