Update vm_mode when rebuilding instance with new image

Rebuilds can be made with a totally different image than what the instance
was originally built with. If an image change also changes from HVM to PV
mode, then vm_mode needs to updated on the instance as well. This also
applies to os_type, architecture and auto_disk_config.

Change-Id: I82f4580f0aab30dd3c1fa4cfc0ca76cc81d487e4
Closes-bug: 1296818
Merge "Update vm_mode when rebuilding instance with new image"