Hyper-V: Implement nova rescue

The Nova rescue command will result in a new temporary instance
being spawned using the same image as the instance to be rescued,
attaching the root disk of the original image as secondary local
disk.

The "unrescue" command will result in the temporary instance
being deleted and the original instance restarted.

Implements: blueprint hyper-v-rescue

Change-Id: I6059ae35a77d675f54b98b2b43b5762e1d24365b