Only use boot=on on non-ide disks only (KVM)

boot=on implies that KVM boots using extboot. This is only required
to boot non-IDE disks and has the side-effect that there is at most
one bootable device. This behaviour breaks some operating systems,
most notably the windows installer that tries to chainload the hard-disk
from the CD-ROM after the initial part of the installation.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>