Basically the 'boot=on' boot selection device is something present in
KVM but not in upstream QEmu, as a result if we boot a QEmu domain
without KVM acceleration we must disable boot=on ... even if the front
end kvm binary expose that capability in the help page.

* src/qemu/qemu_conf.c: in qemudBuildCommandLine if -no-kvm
  is passed, then deactivate QEMUD_CMD_FLAG_DRIVE_BOOT