xenapi: Improve test_xenapi unit testing performance

In investigating performance of the test_xenapi unit testing I
discovered that about 25% of the time is spent in FakeVirtAPI.instance_update.
This function in turn calls db.instance_update_and_get_original to note changes
in state during spawn for most of the unit testing in XenAPIVMTestCase.

XenAPIVMTestCase, doesn't ever check/verify the updating of db based on spawning
of instances.  As such this is wasted effort which can be eliminated since that
verificaiton is not the intent of the tests from what I can gather.

In my testing this reduced running times from 19-20 seconds down to 14-15 seconds
with all unit tests still succeeding. test_xenapi is the by and far the largest
offender in xenapi tests.  Overall xenapi tests we're reduce from
21-22 seconds to 16-17 seconds.

Fixes bug 1218012

Change-Id: Iaa6441655ad100fdb4ffd2bd5eb6265429c133f6
Merge "xenapi: Improve test_xenapi unit testing performance"