volume: fix constants in validateCreateVolumeParams

This patch solves a problem related to the use of the volume constants.
The issue, introduced in 9d15a8c, was hid by subsequent patches in the
same series.

Change-Id: I2d809f51f68935e3c1d4670ee1556e262dd1b10f
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28250