Remove uses of instance['instance_type'] from xenapi driver.

Note that this includes a fix to tests/db/fakes.py to make the fake
instance_type structures look like the database models.

This is one change in a series aimed at removing the use of instance-linked
instance_type objects, in favor of the decoupled type data in
system_metadata. See bug 1140119 for more details.

Change-Id: Ieb107fbaf0f3b5a42681aefc0ffdefdb8c0f98a5
Merge "Remove uses of instance['instance_type'] from xenapi driver."