Add helper method for checking if VM is booting from a volume

Add a _is_booted_from_value method which determines whether
the disk mapping indicates that the VM is booting from a volume
Update the _create_image method to use this helper API.

Related-Bug: #1271528
Change-Id: Iaa92f84ad3de9e84ba49bb255add043f3c8e4c8f
Merge "Add helper method for checking if VM is booting from a volume"