Revert "restapi: Fix NPE in request for CDROMs of wrong VM"

This reverts part of commit 8d5aa3646e2402a48f88a0532a70d9bb50347e57, as it
breaks listing of empty collections of VM devices.

Change-Id: I14333ee4b8f43da4da287a70304107b225990c44
Bug-Url: https://bugzilla.redhat.com/1159642
Related-To: https://bugzilla.redhat.com/1150953
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>
(cherry picked from commit 25215cefbd56b4d9b03d9622f886f762d2570fae)