oop: rename createSparseFile to truncateFile

Change-Id: I832313645aaa33c8d269f8154bd6b0f7ca3946ea
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/14588
Reviewed-by: Dan Kenigsberg <danken@redhat.com>