fileVolume: avoid full size truncation for cow

During the creation of a new file volume we are supposed to truncate the
file to its full capacity only if the format is raw. In the cow case we
should just prepare an empty placeholder to be initialized later on by
qemu-img.

Change-Id: I3414459797236b1055d92f33d86baf1fae7f52fd
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27544
Reviewed-by: Dan Kenigsberg <danken@redhat.com>