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
(cherry picked from commit 1f1414776c73448f68e74f0bcb037329640e38a1)

Signed-off-by: Haomai Wang <haomai@unitedstack.com>
Change-Id: I3ec53b3617d52f75784ebb3b0dad92ca815f8876