core: fixing restore.sh log file location

Setting default log file to be under /var/log/ovirt-engine if this
directory exists

Change-Id: I3cd0742896e2fb5cfa79c75147ced47b94c37155
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1030834
Signed-off-by: Eli Mesika <emesika@redhat.com>