vdsm: prepareVolumePath correct path handling for cdrom

prepareVolumePath looked for 'path' key in specParams
without looking at path in device itself, causing empty
path in specParams to override device path. This patch
adds device.path to checking mechanism to avoid overriding
valid path

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1009469
Change-Id: I10e5e0c12e7cc37a692c0e5c47eed3e4c8e7cda9
Signed-off-by: Martin Polednik <mpoledni@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/20074
Reviewed-by: Michal Skrivanek <michal.skrivanek@redhat.com>
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>