vm: Fix error with missing cdrom in conf

Avoids referencing the cdrom parameter in conf in case it is missing.

Change-Id: Idfebf61316372f24428f7342479a5f29637a4fc7
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1145728
Signed-off-by: Vitor de Lima <vdelima@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/33501
Reviewed-by: Dan Kenigsberg <danken@redhat.com>