Fixes rbd backend image size

The original fix for bug 1219658 introduced a factor of 1024 error
in the resulting rbd image size.

Fix bug #1219658

Change-Id: I3ec53b3617d52f75784ebb3b0dad92ca815f8876
Co-authored-by: Nikola Dipanov <ndipanov@redhat.com>
Signed-off-by: Haomai Wang <haomai@unitedstack.com>
(cherry picked from commit f27ca3134743b45e55afd6c73abbb83c87ba8489)
Merge "Fixes rbd backend image size" into stable/icehouse