Fix cache delete-all-queued-images for xattr

Deleting all queued images was broken when using caching with the xattr
driver, due to a missing get_cache_files function.

The missing function can be satisfied by substituting the call to
get_cache_files() with get_all_regular_files()

Fixes bug 1197402

Change-Id: Ia7f289e15615ef31865ebfd23902aa1ae1b05d66
Merge "Fix cache delete-all-queued-images for xattr"