backend: upgrade - process symbolic link files

Currently if users decided to upgrade the ovirt-node via restapi using
the symbolic link name ovirt-node-iso.iso it will fail. This patch adds
getCanonicalPath() call to resolves the symbolic links.

Change-Id: If4084095c40848dc5573d434c1c7ca497883b280
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1093303
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>