[Libvirt-cim] [PATCH 0 of 5] [TEST] Rework negative tc to not use try_getinstance()

try_getinstance() really isn't needed anymore as the same functionality can be achieved using the functions in enumclass.py. Removing try_getinstance() requires essentially a re-write of these tests - they're old, so updates are needed anyway. More test re-writes will follow in the next set. As these are re-writest, it's easier to review the test case with the patch applied.