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.

Change-Id: Iff3c5a34dd0fe6cfb6b4ab3bc77d34c4c0b57939
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1149637
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1075102
Signed-off-by: Vitor de Lima <vitor.lima@eldorado.org.br>
Signed-off-by: Omer Frenkel <ofrenkel@redhat.com>