KVM virtual machine boot from CD

<os>
  <type>hvm</type>
  <loader>/usr/lib/xen/boot/hvmloader</loader>
  <boot dev='network'/>
  <boot dev='cdrom'/>
  <boot dev='hd'/>
  <bootmenu enable='yes'/>
</os>

Which means the boot order is:

    boot from network like PXE boot
    boot from CD-ROM if no PXE boot
    boot from local hard disk if no PXE boot nor bootable CD
After adding the above configuration, F12 can choose the boot order