VMware: Unit test refactoring

VMware: Unit test refactoring

Some of the unit tests use mox instead of mock. Also, there are
cases where a single test tests multiple cases and methods. This
patch refactors the unit tests for the following methods in the
vmdk module to fix these issues:

* get_volume_stats
* _verify_volume_creation
* create_volume
* delete_volume
* _get_extra_spec_disk_type
* _get_disk_type
* create_snapshot
* delete_snapshot

There will be follow-up patches to fix the remaining unit tests.

Partial-bug: #1261097
Change-Id: I17b1c0df4e13bf9900a2fe8d37ff5297a8e7486e