tests: validate cdrom path sent to/received from vdsm

related to http://gerrit.ovirt.org/#/c/20074/

Tests prepareVolumePath for cdrom by verifying that path and
specParams['path'] and vmPayload works as expected.

Change-Id: I620c481056f7ee3b4d3270ba1f4bc45cbb132ebb
Signed-off-by: Martin Polednik <mpoledni@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/22194
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>