volume: add the BLOCK_SIZE constant

Change-Id: Ia517ec858fb0d7ee45323b7e2a3584c0858222f0
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/14589
Reviewed-by: Dan Kenigsberg <danken@redhat.com>