storageServer: Minor cleanup

Various cleanups, addressing Federico and Dan comments from the previous
patch:

- Make match() private
- Replace str(e) with "%s" % e, to avoid (unlikely) unicode encoding
  errors when converting to string.
- Use "mismatches" instead of "mismatch"

Change-Id: I6589c3af30b5f8ddd69ecdf397864fcb34e561cd
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/36228
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>