core: NPE on UpdateVmCommand when deleting a cdrom from a VM using REST

The problem was that the getParameters().isBalloonEnabled() is a Boolean and
was not set (e.g. it was null) causing a NPE on autoboxing to boolean in
checkPciAndIdeLimit().

Fixed the same way as all the other devices are done (e.g.
isVirtioScsiEnabled()).

Change-Id: Ib9f9d6e47dd90d2501b43587532235d14928aa64
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>