Get the size of the real iso file not the link file

ovirt-node-image.iso is a link file and iso size need
to get from the real iso file via "ls".

Change-Id: I5bd39f86805b0ec3ead5b41d3d0623f49831ca32
Signed-off-by: Wenyi Gao <wenyi@linux.vnet.ibm.com>