Add volume snapshot delete API test case

Later in this patch series I'm adding some additional functionality to
the snapshot delete API.  I noticed that there was no unit tests for the
API.  Go ahead and add a base test case now that I can add more to
later.

Related to blueprint qemu-assisted-snapshots

Change-Id: Iff4e4c0e7de48d5375d66271dfc8ad4cbdff2a31
Merge "Add volume snapshot delete API test case"