Updating powervm driver snapshot with update_task_state flow

Fixes bug 1136359

Without this change snapshot still works but instance task state
does not get updated correctly and nova logs will show expected
task state erros from compute manager.

Also added a snapshot test.

Change-Id: I0f4ed24aa82a66fafc4b6813d017618ae07b4751
Merge "Updating powervm driver snapshot with update_task_state flow"