7.12. Boot Devices
Each virtual machine boots from a selected device. The
boot_devices element lists such boot_device options.
<capabilities>
<version major="3" minor="3">
...
<boot_devices>
<boot_device>cdrom</boot_device>
<boot_device>hd</boot_device>
<boot_device>network</boot_device>
</boot_devices>
...
</version>
</capabilities>