hsm:  deleteImage - fix string format

getExclusiveLock -> namespace should be formatted
as '"%s_%s" % (imgUUID, sdUUID)'.

Change-Id: Ia3b7b491ba2ccb695c6f555d18007a6d50054349
Signed-off-by: Daniel Erez <derez@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/29666
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>