Add helper method for injecting data in an image

Add a _inject_data method whish helps to injecting data
in a disk image file system.
Update the _create_image method to use this helper

Change-Id: Ia005eee0a34d77bbf3eb33f7fef231dcf1eabe2c
Related-Bug: #1271528
Merge "Add helper method for injecting data in an image"