Fix typo in PowerPC plugin

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>
Break up exception handling in FileCacheArchive.add_file()

An exception can occur at several points in add_file()

- Copying the source to the destination
- Propagating permissions with shutil.copystat
- Setting ownership via os.chown

The second of these can occur when copying SELinux xattrs from
/proc. Separate out the three cases and ignore failures in
copystat.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>