Implement hard reboot for powervm driver

Implements the OS reboot API in the powervm driver. Soft reboot is not
supported as it requires additional configuration on the LPAR when it's
spawned so this patch simply performs an immediate hard reboot.

Also adds tests for start_lpar and stop_lpar which didn't exist before.

Closes-Bug: #1205089

Change-Id: Iad64dda79f46d18c7aa5bee12ed41e7a01853d34
Merge "Implement hard reboot for powervm driver"