Create canonical unit testing InstanceClient #130

src/controller.rs has:

• VcpuLimitedClient
• BulkClient
• SnapshotClient
• FailingClient

src/instance.rs:

• SnapshotClient
• FailingClient

And they all look very similar.

It would be nice to have a single version of this which can act as most, if not all, of these. Can FakeBackend (in #78) stand in for this or should it be a new InstanceClient?