Don't inject files while resizing instance

Fixes bug 1208680

As what the code owner of method 'finish_migration' in libvirt driver
said, the files injecting operation is not necessary during resize,
because they have already been injected when creating the instance,
so added a parameter to _create_image to fix this issue, which is used
for identifying the injection is needed or not.

Change-Id: I01990de0ad2c7f1c2e499144034b7a4ac9faf024
Merge "Don't inject files while resizing instance"