Rescue should give guests a chance to shutdown

Currently in libvirt operations which power off an instance such as stop,
shelve, rescue, and resize simply destroy the underlying VM. Some
GuestOS's do not react well to this type of power failure, and so it would
be better if these operations followed the same approach as soft_reboot
and give the GuestOS as chance to shutdown gracefully.

This is one of a set of changes that will eventually expose the choice
of whether to give the GuestOS a chance to shutdown via the API.

This change implements the compute manager changes to rescue.

Partially-Implements: blueprint user-defined-shutdown
DocImpact

Change-Id: I296397f4fe3308be3727ec127929cea60d46b882
Merge "Rescue should give guests a chance to shutdown"