Correct space calculation for backup

The test should be numeric and not
lexographical.

Change-Id: I0852717f09277d9e4de7f55a7edcea2bed22835d
Closes-Bug: #1375810
(cherry picked from commit 4fc37f1caf498bcde0c6943af01d64271df79225)
Merge "Correct space calculation for backup" into stable/5.1