We use stack.prepare_abandon() to make the
snapshot data, but the data have no 'files' then will
result in ValueError raised(No content found in the "files"
section for get_file path) when restore.
This patch will store 'files' for snapshot data(
also for prepare abandon data), then we can get 'files'
from the snapshot data when stack restore.
Change-Id: I0fddf92e950b93dce70eb8e5a562cf8d0046a9ab
Closes-Bug: #1461375
8140409e63