core, engine: Avoid creating the CD device multiple times

When there are more than one device attached to the blank template, the
AddVmCommand creates multiple CD devices which conflict with each other.
This change fixes this issue, but still assumes that the blank template
has at least one device attached to it.

This patch has already been pushed to 3.4
(Iff3c5a34dd0fe6cfb6b4ab3bc77d34c4c0b57939) but was abandoned, now it seems to
be indeed relevant so pushing again under a different change id.

Change-Id: I34c0ea6d32ca378f22b9309b4c532d65c59337ba
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1075102
Signed-off-by: Vitor de Lima <vitor.lima@eldorado.org.br>