Do not extend volume beyond truesize

When calling getNextVolumeSize on a volume that is close to fully
allocated, the function can return a value larger than the volume's real
capacity.  Add an explicit ceiling to prevent this.

Change-Id: Ib616224d88c886700a7f121eaf6da84ba8276e55
Signed-off-by: Adam Litke <alitke@redhat.com>
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1155583
Reviewed-on: http://gerrit.ovirt.org/35757
Reviewed-by: Allon Mureinik <amureini@redhat.com>
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>