KVM: strip boot=on from IDE CDROM drivers

boot=on forces KVM to use an extboot ROM which has shown poor behaviour with
booting from CD images. IDE CDROM emulation (which is forced by default when
booting from CD) does not need boot=on, so we simply omit it.

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>