[xenapi] Unshadow an important test case class

A recent commit added a shadow of the VMOpsTestCase test class in
nova/tests/virt/xenapi/test_vmops.py.  This renames the class to
eliminate the shadowing.  It also removes an unnecessary setUp()
method.

Fixes bug 1206703.

Change-Id: I6f36cea7d926c5773cfee1c5868c72acf56f3220
Merge "[xenapi] Unshadow an important test case class"